diff --git a/.gitignore b/.gitignore
index b803840d09e5..3184a481591e 100644
--- a/.gitignore
+++ b/.gitignore
@@ -23,29 +23,11 @@ Thumbs.db
 /Source/Workers/**
 !/Source/Workers/cesiumWorkerBootstrapper.js
 !/Source/Workers/transferTypedArrayTest.js
+!/Source/Workers/package.json
 
 Source/ThirdParty/_commonjsHelpers*
-Source/ThirdParty/Autolinker.js
-Source/ThirdParty/bitmap-sdf.js
-Source/ThirdParty/dompurify.js
-Source/ThirdParty/earcut.js
 Source/ThirdParty/draco_decoder.wasm
 Source/ThirdParty/Workers/draco_decoder_nodejs.js
-Source/ThirdParty/grapheme-splitter.js
-Source/ThirdParty/jsep.js
-Source/ThirdParty/kdbush.js
-Source/ThirdParty/ktx-parse.js
-Source/ThirdParty/lerc.js
-Source/ThirdParty/mersenne-twister.js
-Source/ThirdParty/meshoptimizer.js
-Source/ThirdParty/nosleep.js
-Source/ThirdParty/pako.js
-Source/ThirdParty/protobufjs.js
-Source/ThirdParty/rbush.js
-Source/ThirdParty/topojson.js
-Source/ThirdParty/Tween.js
-Source/ThirdParty/Uri.js
-Source/ThirdParty/zip.js
 Source/ThirdParty/Workers/pako_inflate.min.js
 Source/ThirdParty/Workers/pako_deflate.min.js
 Source/ThirdParty/Workers/z-worker-pako.js
diff --git a/.travis.yml b/.travis.yml
index fb79557cca9b..25eb5951a094 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -12,14 +12,14 @@ script:
   - npm --silent run markdownlint
   - npm --silent run prettier-check
 
-  - npm --silent run build
+  - npm --silent run build -- --node
+
   - npm --silent run coverage -- --browsers FirefoxHeadless --webgl-stub --failTaskOnError --suppressPassed
 
   - travis_wait 20 npm --silent run makeZipFile -- --concurrency 1
   - npm pack &> /dev/null
 
   - npm --silent run buildApps
-  - npm --silent run build-specs
 
   - npm --silent run deploy-s3 -- -b cesium-dev -d cesium/$TRAVIS_BRANCH --confirm -c 'no-cache'
   - npm --silent run deploy-status -- --status success --message Deployed
@@ -28,7 +28,7 @@ script:
 
   # Various Node.js smoke-screen tests
   - node -e "const Cesium = require('./');"
-  - NODE_ENV=development node index.cjs
-  - NODE_ENV=production node index.cjs
+  - NODE_ENV=development node Specs/test.cjs
+  - NODE_ENV=production node Specs/test.cjs
 
   - npm --silent run cloc
diff --git a/Apps/CesiumViewer/CesiumViewer.js b/Apps/CesiumViewer/CesiumViewer.js
index f642804ffc94..2226c85e8fa2 100644
--- a/Apps/CesiumViewer/CesiumViewer.js
+++ b/Apps/CesiumViewer/CesiumViewer.js
@@ -1,4 +1,6 @@
-window.CESIUM_BASE_URL = "../../Source/";
+if (window.CESIUM_BASE_URL === undefined) {
+  window.CESIUM_BASE_URL = "../../Build/CesiumUnminified/";
+}
 
 import {
   Cartesian3,
@@ -16,7 +18,7 @@ import {
   Viewer,
   viewerCesiumInspectorMixin,
   viewerDragDropMixin,
-} from "../../Source/Cesium.js";
+} from "../../Build/CesiumUnminified/index.js";
 
 function main() {
   /*
diff --git a/Apps/CesiumViewer/index.js b/Apps/CesiumViewer/index.js
new file mode 100644
index 000000000000..f4e3e3d56f26
--- /dev/null
+++ b/Apps/CesiumViewer/index.js
@@ -0,0 +1 @@
+window.CESIUM_BASE_URL = ".";
diff --git a/Apps/Sandcastle/load-cesium-es6.js b/Apps/Sandcastle/load-cesium-es6.js
index 8a55b25b6f41..b751b26f1e88 100644
--- a/Apps/Sandcastle/load-cesium-es6.js
+++ b/Apps/Sandcastle/load-cesium-es6.js
@@ -1,7 +1,7 @@
 // This file loads the unbuilt ES6 version of Cesium
 // into the global scope during local developmnet
-window.CESIUM_BASE_URL = "../../../Source/";
-import * as Cesium from "../../Source/Cesium.js";
+window.CESIUM_BASE_URL = "../../../Build/CesiumUnminified/";
+import * as Cesium from "../../Build/CesiumUnminified/index.js";
 window.Cesium = Cesium;
 
 // Since ES6 modules have no guaranteed load order,
diff --git a/Apps/Sandcastle/standalone.html b/Apps/Sandcastle/standalone.html
index 33b837e24fdb..54e348a6c6ec 100644
--- a/Apps/Sandcastle/standalone.html
+++ b/Apps/Sandcastle/standalone.html
@@ -11,11 +11,6 @@
     <script type="text/javascript" src="Sandcastle-header.js"></script>
     <script type="text/javascript" src="Sandcastle-client.js"></script>
     <script type="text/javascript" src="ThirdParty/pako.min.js"></script>
-    <script
-      type="text/javascript"
-      src="../../Build/CesiumUnminified/Cesium.js"
-      nomodule
-    ></script>
     <script type="module" src="load-cesium-es6.js"></script>
     <script type="text/javascript" src="Sandcastle-helpers.js"></script>
     <style>
diff --git a/Apps/Sandcastle/templates/bucket-requirejs.html b/Apps/Sandcastle/templates/bucket-requirejs.html
index d7648fbeff89..bde1e75919fc 100644
--- a/Apps/Sandcastle/templates/bucket-requirejs.html
+++ b/Apps/Sandcastle/templates/bucket-requirejs.html
@@ -9,11 +9,6 @@
     />
     <title>Cesium Demo</title>
     <script type="text/javascript" src="../Sandcastle-header.js"></script>
-    <script
-      type="text/javascript"
-      src="../../../Build/CesiumUnminified/Cesium.js"
-      nomodule
-    ></script>
     <script type="module" src="../load-cesium-es6.js"></script>
   </head>
   <body
diff --git a/CHANGES.md b/CHANGES.md
index 47b7c5188fd9..e9b5396f7d3a 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -5,6 +5,10 @@
 ##### Breaking Changes :mega:
 
 - Tilesets and entities now use `ModelExperimental` by default. This can still be changed by setting `ExperimentalFeatures.enableModelExperimental = false`. [#10530](https://github.com/CesiumGS/cesium/pull/10530)
+- Built `Cesium.js` is no longer AMD format. This may or may not be a breaking change depending on how you use Cesium in your app. See our [blog post](TODO) for the full details. [#10399](https://github.com/CesiumGS/cesium/pull/10399)
+  - If you were ingesting individual ESM-style modules from the combined file `Build/Cesium/Cesium.js` or `Build/CesiumUnminified/Cesium.js`, instead use `Build/Cesium/index.js` or `Build/CesiumUnminified/index.js` respectively.
+  - `CESIUM_BASE_URL` should be set to either `Build/Cesium` or `Build/CesiumUnminfied`.
+  - Built `Cesium.js` has gone from `12.5MB` to `8.4MB` unminified and from `4.3MB` to `3.6MB` minified. `Cesium.js.map` has gone from `22MB` to `17.2MB`.
 
 ##### Additions :tada:
 
diff --git a/Documentation/Contributors/BuildGuide/README.md b/Documentation/Contributors/BuildGuide/README.md
index 3751d3ce0594..7ed0a3cef645 100644
--- a/Documentation/Contributors/BuildGuide/README.md
+++ b/Documentation/Contributors/BuildGuide/README.md
@@ -4,6 +4,8 @@
   - [Quickstart](#quickstart)
   - [Get the Code](#get-the-code)
   - [Build the Code](#build-the-code)
+    - [Development Server](#development-server)
+    - [Build Output](#build-output)
   - [Build Scripts](#build-scripts)
   - [Travis and Continuous Integration](#travis-and-continuous-integration)
     - [Configure a Different S3 Bucket](#configure-a-different-s3-bucket)
@@ -29,8 +31,6 @@
 
 4. Navigate to : [`http://localhost:8080/`](http://localhost:8080)
 
-_NOTE: If you change branches, you might have to rebuild._
-
 ---
 
 ## Get the Code
@@ -55,7 +55,7 @@ _NOTE: If you change branches, you might have to rebuild._
 
 Prerequisites:
 
-- Install [Node.js](http://nodejs.org/) on your system. Building Cesium requires Node 6.x or newer.
+- Install [Node.js](http://nodejs.org/) on your system. Building Cesium requires Node 14.x or newer.
 
 Cesium uses [npm modules](https://docs.npmjs.com/getting-started/what-is-npm) for development, so after syncing, you need to run `npm install` from the Cesium root directory:
 
@@ -69,6 +69,12 @@ Once all modules have been installed, run `npm run build` to actually build the
 npm run build
 ```
 
+Alternatively, if you want to edit source files and see the changes reflected for testing, use `npm run build-watch`.
+
+```bash
+npm run build-watch
+```
+
 Cesium ships with a simple HTTP server for testing, run `npm start` after building to use it:
 
 ```bash
@@ -82,12 +88,7 @@ Then browse to [http://localhost:8080/](http://localhost:8080/). The landing pag
 - **Test Suites** : tests using [Jasmine](https://jasmine.github.io/). [Testing guide here.](https://github.com/CesiumGS/cesium/blob/main/Documentation/Contributors/TestingGuide/README.md#testing-guide)
 - **Documentation** : reference documentation built from source. [Documentation guide here.](https://github.com/CesiumGS/cesium/blob/main/Documentation/Contributors/DocumentationGuide/README.md#documentation-guide)
 
-Cesium can be used in two different ways. Cesium can be either a set of modules using [Asynchronous Module Definition (AMD)](https://github.com/amdjs/amdjs-api/wiki/AMD), or it can be built as one combined file containing all modules. The basics:
-
-- `npm run build` will build AMD Cesium. This also builds Cesium Viewer and Sandcastle.
-- `npm run minifyRelease` creates the built version of Cesium. This also builds Hello World.
-
-Read the complete list of build scripts below for more details.
+### Development Server
 
 By default, the server only allows connections from your local machine. To allow connections from other machines, pass
 the `--public` option to npm. Note the extra `--` is intentional and required by npm.
@@ -96,12 +97,26 @@ the `--public` option to npm. Note the extra `--` is intentional and required by
 npm start -- --public
 ```
 
-The development server has a few other options as well, which you can see by pasing the `--help` parameter:
+The development server has a few other options as well, which you can see by passing the `--help` parameter:
 
 ```bash
 npm start -- --help
 ```
 
+### Build Output
+
+Cesium offers a few different distributions. When developing, make sure to pick the one that fits your app's architecture.
+
+- [IIFE (immediately-invoked function expression)](https://developer.mozilla.org/en-US/docs/Glossary/IIFE) - A pre-processed bundle and optimized for the browser, which defines a `Cesium` global variable upon loading `Build/Cesium/Cesium.js`. While much of our documentation uses IIFE-style globals for ease-of-use, we do not recommend this approach for production apps.
+- [ESM (ECMAScript modules)](https://nodejs.org/api/esm.html) - Standard for packaging JS code which is supported by most browsers and NodeJS. Modules use `import` and `export` statements. Unprocessed, individual modules are available in the `Source` directory, accessible by importing `Source/Cesium.js`; A single pre-processed bundle by importing `Build/Cesium/index.js`.
+- [CJS (CommonJS)](https://nodejs.org/api/modules.html) - A pre-processed, bundled module packaged for running in NodeJS accessible by requiring `index.cjs`.
+
+In much of our documentation, we use IIFE as it can be easily loaded with a CDN and defines a global _Cesium_ variable with all modules attached.
+
+For a production app, we recommend using the `Source` modules directly which will allow your build tool of choice to reduce the final release's size using tree shaking.
+
+Read the complete list of build scripts and options below for more details.
+
 While you can use the editor of your choice to develop Cesium, certain files, such as `glsl` and new tests, require that
 the `build` task be executed in order for the changes to take effect. You can use the `build-watch` script to have this
 happen automatically.
@@ -119,40 +134,43 @@ npm run [target-name]
 Here's the full set of scripts and what they do.
 
 - **Build scripts** -- build and package the source code and documentation
-  - `build` - A fast, developer-oriented build that prepares the source tree for use as standard [Asynchronous Module Definition (AMD)](https://github.com/amdjs/amdjs-api/wiki/AMD) modules, suitable for running tests and most examples (some Sandcastle examples require running `combine`). Run this when a GLSL shader is changed since the .glsl file is converted to a .js file with a string for the GLSL source. This runs automatically when saving files in Eclipse.
-  - `build-watch` - A never-ending task that watches your file system for changes to Cesium and runs `build` on the source code as needed.
-  - `combine` - Runs `build`, plus the [the RequireJS optimizer](http://requirejs.org/docs/optimization.html) to combine Cesium and [the Almond AMD loader](http://requirejs.org/docs/faq-optimization.html#wrap) to produce all-in-one files in the `Build/Cesium` directory that exposes the entire Cesium API attached to a single global `Cesium` object. This version is useful if you don't want to use the modules directly with a standard AMD loader.
-  - `minify` - Runs `combine`, plus [minifies](<http://en.wikipedia.org/wiki/Minification_(programming)>) Cesium.js.
-  - `combineRelease` - Runs `combine`, plus uses the optimizer to remove debugging code that validates function input and throws DeveloperErrors. The removed sections are marked with `//>>includeStart('debug', pragmas.debug);` blocks in the code.
-  - `minifyRelease` - Runs `minify`, and removes debugging code.
-  - `requirejs` - Used internally by the build system and can not be called directly.
-  - `buildApps` - Builds the example applications (such as Cesium Viewer) to produce self-contained, minified, deployable versions in the `Build` directory. This script requires a release build of Cesium, run the `release` script to build one if needed.
-  - `generateDocumentation` - Generates HTML documentation in `Build/Documentation` using [JSDoc 3](https://github.com/jsdoc3/jsdoc). More [details here](https://github.com/rahwang/cesium/tree/main/Documentation/Contributors/DocumentationGuide).
+  - `build` - A fast, developer-oriented build that bundles the source modules to produce all-in-one files in the `Build/CesiumUnminified` directory that exposes the entire Cesium API attached to a single global `Cesium` object. Run this when a GLSL shader is changed since the .glsl file is converted to a .js file with a string for the GLSL source.
+    - `--minify` - [Minifies](<http://en.wikipedia.org/wiki/Minification_(programming)>) the output for optimized loading. Specifying this option will output to `Build/Cesium`.
+    - `--removePragmas` - Optimizes the output by removing debugging code that validates function input and throws `DeveloperError`s. The removed sections are marked with `//>>includeStart('debug', pragmas.debug);` blocks in the code.
+    - `--node` - Bundles an `index.cjs` module targeted for use in NodeJS
+  - `build-watch` - A never-ending task that watches your file system for changes to Cesium and builds the source code as needed. All `build` options are also available for this task.
+  - `minify` - Bundles the source modules and [minifies](<http://en.wikipedia.org/wiki/Minification_(programming)>) to produce an all-in-one files in the `Build/Cesium` directory.
+  - `combineRelease` - Bundles plus removes debugging code that validates function input and throws DeveloperErrors. The removed sections are marked with `//>>includeStart('debug', pragmas.debug);` blocks in the code.
+  - `minifyRelease` - Bundles, minifies, and removes debugging code.
+  - `buildApps` - Builds the example applications (such as Cesium Viewer) to produce self-contained, minified, deployable versions in the `Build` directory.
+  - `generateDocumentation` - Generates HTML documentation in `Build/Documentation` using [JSDoc 3](https://github.com/jsdoc3/jsdoc). See the [Documentation Guide](../DocumentationGuide/README.md) for more details.
+  - `build-ts` - Generates a TypeScript definitions file for the Cesium library
+  - `build-third-party` - Generates `ThirdParty.json`, a file which lists the latest licensing information of installed third party modules
   - `release` - A full release build that creates a shippable product, including generating documentation.
-  - `makeZipFile` - Builds a zip file containing all release files. This includes the source tree (suitable for use from an AMD-aware application), plus the combined and minified Cesium.js files, the generated documentation, the test suite, and the example applications (in both built and source form).
+  - `makeZipFile` - Builds a zip file containing all release files. This includes the source ESM modules, bundled ESM and IIFE format `Cesium.js`, plus the bundled minified versions of ESM and IIFE, the generated documentation, the test suite, and the example applications (in both built and source form).
 - **Utility scripts** -- code coverage, static code analysis, and other utilities
-  - `coverage` - Runs coverage and opens the default browser with the results.
-  - `eslint` - Runs [ESLint](http://eslint.org/), a static code analysis tool, on the entire source tree.
-  - `eslint-watch` - A never-ending task that watches your file system for changes to Cesium and runs ESLint on any changed source files.
-  - `markdownlint` - Runs [markdownlint](https://github.com/DavidAnson/markdownlint) on the Markdown files used for documentation.
-  - `clean` - Removes all generated build artifacts.
+  - `clean` - Removes all generated build artifacts
   - `cloc` - Runs [CLOC](https://github.com/AlDanial/cloc) to count the lines of code on the Source and Specs directories. This requires [Perl](http://www.perl.org/) to execute.
+  - `coverage` - Runs coverage and opens the default browser with the results
+  - `eslint` - Runs [ESLint](http://eslint.org/), a static code analysis tool, on the entire source tree
+  - `prettier` - Formats the code base using [Prettier](https://prettier.io/)
+  - `prettier-check` - Verifies prettier formatting, but does not write the output
 - **Testing scripts** -- build and run the unit tests
   - `test` - Runs all tests with [Karma](http://karma-runner.github.io/0.13/index.html) using the default browser specified in the Karma config file.
-  - `test-all` - Runs all tests with Karma using all browsers installed on the current system.
-  - `test-non-webgl` - Runs only non-WebGL tests.
-  - `test-webgl` - Runs only WebGL tests.
-  - `test-webgl-stub` - Runs all tests using the WebGL stub, which WebGL calls a noop and ignores related test expectations.
-  - `test-webgl-validation` - Runs all tests with Karma and enables low-level WebGL validation.
-  - `test-release` - Runs all tests on the minified release version of built Cesium.
+  - `test-all` - Runs all tests with Karma using all browsers installed on the current system
+  - `test-non-webgl` - Runs only non-WebGL tests
+  - `test-webgl` - Runs only WebGL tests
+  - `test-webgl-stub` - Runs all tests using the WebGL stub, which WebGL calls a noop and ignores related test expectations
+  - `test-webgl-validation` - Runs all tests with Karma and enables low-level WebGL validation
+  - `test-release` - Runs all tests on the minified release version of built Cesium
 - **Deployment scripts**
-  - `deploy-s3` - Deploys the built CesiumJS files, the npm package, and the zip file to Amazon S3. This requires having credentials set up for the S3 bucket to which you are deploying.
-  - `deploy-status` - Set the deployment statuses in GitHub, for use with Travis.
-  - `deploy-set-version` - Sets the version of `package.json`, for use with Travis.
+  - `deploy-s3` - Deploys the built CesiumJS files, the npm package, and the zip file to Amazon S3. This requires having credentials set up for the S3 bucket to which you are deploying
+  - `deploy-status` - Sets the deployment statuses in GitHub, for use with Travis
+  - `deploy-set-version` - Sets the version of `package.json`, for use with Travis
 
 ## Travis and Continuous Integration
 
-Cesium uses [Travis](https://travis-ci.com/) for continuous integration. The Travis configuration and all the steps of the build process are defined in `travis.yml`. The blog post [Cesium Continuous Integration](http://cesium.com/blog/2016/04/07/Cesium-Continuous-Integration/) contains an in-depth explaination of the travis build process.
+Cesium uses [Travis](https://travis-ci.com/) for continuous integration. The Travis configuration and all the steps of the build process are defined in `travis.yml`. The blog post [Cesium Continuous Integration](http://cesium.com/blog/2016/04/07/Cesium-Continuous-Integration/) contains an in-depth explanation of the travis build process.
 
 Travis triggers a build whenever someone opens a pull request or pushes code to the Cesium repository. After the build has completed, at the bottom on the pull request, the status of the build is shown and you can access the build by clicking the "Details" link.
 
@@ -200,7 +218,7 @@ To configure the additional commit statuses on GitHub for your fork of Cesium, y
 
 - In GitHub, go to "Settings" and "Personal access tokens"
 - Click "Generate new token" and confirm your password when prompted
-- Give a breif description of the token such as "Travis Statuses" and select "repo:status" and click "Generate token"
+- Give a brief description of the token such as "Travis Statuses" and select "repo:status" and click "Generate token"
   - `repo:status` gives the token access to only commit statuses
 
 ![Token Access](token.jpg)
diff --git a/Documentation/Contributors/TestingGuide/README.md b/Documentation/Contributors/TestingGuide/README.md
index 209ebfc3e5b4..315090ec872d 100644
--- a/Documentation/Contributors/TestingGuide/README.md
+++ b/Documentation/Contributors/TestingGuide/README.md
@@ -106,9 +106,9 @@ These tests run quickly (for example, 15 seconds compared to 60) and are very re
 
 #### Run All Tests Against the Minified Release Version of CesiumJS
 
-Most test options load CesiumJS using the individual source files in the `Source` directory, which is great for debugging.
+Most test options load CesiumJS with the unminified build plus a source map, which is great for debugging.
 
-However, many users build apps using the built Cesium.js in `Build/Cesium` (which is created, for example, by running `npm run minifyRelease`). This option runs the tests using this instead of individual CesiumJS source files. The release version has `DeveloperError` exceptions optimized out so this test option makes `toThrowDeveloperError` always pass. See the [Build Guide](https://github.com/CesiumGS/cesium/blob/main/Documentation/Contributors/BuildGuide/README.md#build-scripts) for all the CesiumJS build options. When testing against built Cesium.js, the specs need to be built as well with `npm run build-specs`.
+However, many users build apps using the built Cesium.js in `Build/Cesium` (which is created, for example, by running `npm run release`). This option runs the tests using this instead of the unminified build. The release version has `DeveloperError` exceptions optimized out so this test option makes `toThrowDeveloperError` always pass. See the [Build Guide](https://github.com/CesiumGS/cesium/blob/main/Documentation/Contributors/BuildGuide/README.md#build-scripts) for all the CesiumJS build options.
 
 `npm run test-release`
 
@@ -188,11 +188,11 @@ These tests run quickly (for example, 15 seconds compared to 60) and are very re
 
 #### Run All Tests against Combined File (Run All Tests against Combined File with Debug Code Removed)
 
-Most test options load CesiumJS using the individual source files in the `Source` directory, which is great for debugging.
+Most test options load CesiumJS with the unminified build plus a source map, which is great for debugging.
 
-However, many users build apps using the built Cesium.js in `Build/Cesium` (which is created, for example, by running `npm run combine`). This option runs the tests using this instead of individual CesiumJS source files.
+However, many users build apps using the built Cesium.js in `Build/Cesium` (which is created, for example, by running `npm run release`). This option runs the tests using this instead of the unminified build.
 
-The **Run All Tests against Combined File with Debug Code Removed** is the same except it is for use with the release version of the built Cesium.js (which is created, for example, by running `npm run combineRelease`). The release version has `DeveloperError` exceptions optimized out so this test option makes `toThrowDeveloperError` always pass.
+The **Run All Tests against Combined File with Debug Code Removed** is the same except it is for use with the release version of the built Cesium.js (which is created, for example, by running `npm run release`). The release version has `DeveloperError` exceptions optimized out so this test option makes `toThrowDeveloperError` always pass.
 
 See the [Build Guide](https://github.com/CesiumGS/cesium/blob/main/Documentation/Contributors/BuildGuide/README.md#build-scripts) for all the CesiumJS build options.
 
diff --git a/ThirdParty/npm/Autolinker.js b/Source/ThirdParty/Autolinker.js
similarity index 100%
rename from ThirdParty/npm/Autolinker.js
rename to Source/ThirdParty/Autolinker.js
diff --git a/ThirdParty/npm/Tween.js b/Source/ThirdParty/Tween.js
similarity index 100%
rename from ThirdParty/npm/Tween.js
rename to Source/ThirdParty/Tween.js
diff --git a/ThirdParty/npm/Uri.js b/Source/ThirdParty/Uri.js
similarity index 100%
rename from ThirdParty/npm/Uri.js
rename to Source/ThirdParty/Uri.js
diff --git a/Source/ThirdParty/Workers/package.json b/Source/ThirdParty/Workers/package.json
new file mode 100644
index 000000000000..0292b9956f2e
--- /dev/null
+++ b/Source/ThirdParty/Workers/package.json
@@ -0,0 +1 @@
+{"type":"commonjs"}
\ No newline at end of file
diff --git a/ThirdParty/npm/bitmap-sdf.js b/Source/ThirdParty/bitmap-sdf.js
similarity index 100%
rename from ThirdParty/npm/bitmap-sdf.js
rename to Source/ThirdParty/bitmap-sdf.js
diff --git a/ThirdParty/npm/dompurify.js b/Source/ThirdParty/dompurify.js
similarity index 100%
rename from ThirdParty/npm/dompurify.js
rename to Source/ThirdParty/dompurify.js
diff --git a/ThirdParty/npm/earcut.js b/Source/ThirdParty/earcut.js
similarity index 100%
rename from ThirdParty/npm/earcut.js
rename to Source/ThirdParty/earcut.js
diff --git a/ThirdParty/npm/grapheme-splitter.js b/Source/ThirdParty/grapheme-splitter.js
similarity index 100%
rename from ThirdParty/npm/grapheme-splitter.js
rename to Source/ThirdParty/grapheme-splitter.js
diff --git a/ThirdParty/npm/jsep.js b/Source/ThirdParty/jsep.js
similarity index 100%
rename from ThirdParty/npm/jsep.js
rename to Source/ThirdParty/jsep.js
diff --git a/ThirdParty/npm/kdbush.js b/Source/ThirdParty/kdbush.js
similarity index 100%
rename from ThirdParty/npm/kdbush.js
rename to Source/ThirdParty/kdbush.js
diff --git a/ThirdParty/npm/ktx-parse.js b/Source/ThirdParty/ktx-parse.js
similarity index 100%
rename from ThirdParty/npm/ktx-parse.js
rename to Source/ThirdParty/ktx-parse.js
diff --git a/ThirdParty/npm/lerc.js b/Source/ThirdParty/lerc.js
similarity index 100%
rename from ThirdParty/npm/lerc.js
rename to Source/ThirdParty/lerc.js
diff --git a/ThirdParty/npm/mersenne-twister.js b/Source/ThirdParty/mersenne-twister.js
similarity index 100%
rename from ThirdParty/npm/mersenne-twister.js
rename to Source/ThirdParty/mersenne-twister.js
diff --git a/ThirdParty/npm/meshoptimizer.js b/Source/ThirdParty/meshoptimizer.js
similarity index 100%
rename from ThirdParty/npm/meshoptimizer.js
rename to Source/ThirdParty/meshoptimizer.js
diff --git a/ThirdParty/npm/nosleep.js b/Source/ThirdParty/nosleep.js
similarity index 100%
rename from ThirdParty/npm/nosleep.js
rename to Source/ThirdParty/nosleep.js
diff --git a/ThirdParty/npm/pako.js b/Source/ThirdParty/pako.js
similarity index 100%
rename from ThirdParty/npm/pako.js
rename to Source/ThirdParty/pako.js
diff --git a/ThirdParty/npm/protobufjs.js b/Source/ThirdParty/protobufjs.js
similarity index 100%
rename from ThirdParty/npm/protobufjs.js
rename to Source/ThirdParty/protobufjs.js
diff --git a/ThirdParty/npm/rbush.js b/Source/ThirdParty/rbush.js
similarity index 100%
rename from ThirdParty/npm/rbush.js
rename to Source/ThirdParty/rbush.js
diff --git a/ThirdParty/npm/topojson.js b/Source/ThirdParty/topojson.js
similarity index 100%
rename from ThirdParty/npm/topojson.js
rename to Source/ThirdParty/topojson.js
diff --git a/ThirdParty/npm/zip.js b/Source/ThirdParty/zip.js
similarity index 100%
rename from ThirdParty/npm/zip.js
rename to Source/ThirdParty/zip.js
diff --git a/Source/Workers/package.json b/Source/Workers/package.json
new file mode 100644
index 000000000000..0292b9956f2e
--- /dev/null
+++ b/Source/Workers/package.json
@@ -0,0 +1 @@
+{"type":"commonjs"}
\ No newline at end of file
diff --git a/Specs/.eslintrc.json b/Specs/.eslintrc.json
index c564feac0b4f..413d95e351eb 100644
--- a/Specs/.eslintrc.json
+++ b/Specs/.eslintrc.json
@@ -10,7 +10,8 @@
     "overrides": [
         {
             "files": [
-                "karma.conf.cjs"
+                "karma.conf.cjs",
+                "test.cjs"
             ],
             "parserOptions": {
                 "sourceType": "script"
diff --git a/Specs/Core/DeveloperErrorSpec.js b/Specs/Core/DeveloperErrorSpec.js
index 44bf02afc005..bc588f7df7bc 100644
--- a/Specs/Core/DeveloperErrorSpec.js
+++ b/Specs/Core/DeveloperErrorSpec.js
@@ -18,22 +18,21 @@ describe("Core/DeveloperError", function () {
   });
 
   it("has a stack property", function () {
-    if (window.specsUsingRelease) {
-      expect(e.stack).toContain("Specs.js");
-    } else {
-      expect(e.stack).toContain("DeveloperErrorSpec.js");
+    // Since we are using source maps, we won't be able to map to a specific file without help from the browser developer tools.
+    // However, we should know the class if not minified
+    if (!window.specsUsingRelease) {
+      expect(e.stack).toContain(name);
     }
   });
 
   it("has a working toString", function () {
     const str = new DeveloperError(testMessage).toString();
 
-    expect(str).toContain(`${name}: ${testMessage}`);
-
-    if (window.specsUsingRelease) {
-      expect(str).toContain("Specs.js");
+    if (!window.specsUsingRelease) {
+      expect(str).toContain(`${name}: ${testMessage}`);
     } else {
-      expect(str).toContain("Core/DeveloperErrorSpec.js");
+      // Since source maps are used, there will not be exact filenames
+      expect(str).toContain(testMessage);
     }
   });
 });
diff --git a/Specs/Core/RuntimeErrorSpec.js b/Specs/Core/RuntimeErrorSpec.js
index df90ec70edb2..ea1581bbf695 100644
--- a/Specs/Core/RuntimeErrorSpec.js
+++ b/Specs/Core/RuntimeErrorSpec.js
@@ -18,22 +18,21 @@ describe("Core/RuntimeError", function () {
   });
 
   it("has a stack property", function () {
-    if (window.specsUsingRelease) {
-      expect(e.stack).toContain("Specs.js");
-    } else {
-      expect(e.stack).toContain("RuntimeErrorSpec.js");
+    // Since we are using source maps, we won't be able to map to a specific file without help from the browser developer tools.
+    // However, we should know the class if not minified
+    if (!window.specsUsingRelease) {
+      expect(e.stack).toContain(name);
     }
   });
 
   it("has a working toString", function () {
     const str = new RuntimeError(testMessage).toString();
 
-    expect(str).toContain(`${name}: ${testMessage}`);
-
-    if (window.specsUsingRelease) {
-      expect(str).toContain("Specs.js");
-    } else {
-      expect(str).toContain("Core/RuntimeErrorSpec.js");
+    if (!window.specsUsingRelease) {
+      expect(e.stack).toContain(name);
     }
+
+    // Since source maps are used, there will not be exact filenames
+    expect(str).toContain(testMessage);
   });
 });
diff --git a/Specs/SpecRunner.html b/Specs/SpecRunner.html
index c62fdb45e55a..d2883e47d4f8 100644
--- a/Specs/SpecRunner.html
+++ b/Specs/SpecRunner.html
@@ -9,24 +9,19 @@
     <script src="./jasmine/jasmine.js"></script>
     <script src="./jasmine/jasmine-html.js"></script>
     <script src="./jasmine/boot0.js"></script>
-    <script type="module" src="./spec-main.js"></script>
-    <script src="./jasmine/boot1.js"></script>
   </head>
   <body>
     <script>
-      if (window.location.search.indexOf("built") !== -1) {
-        if (window.location.search.indexOf("release") !== -1) {
-          document.write('<script src="../Build/Cesium/Cesium.js"><\/script>');
-        } else {
-          document.write(
-            '<script src="../Build/CesiumUnminified/Cesium.js"><\/script>'
-          );
-        }
-        document.write('<script src="../Build/Specs/Specs.js"><\/script>');
+      if (window.location.search.indexOf("release") !== -1) {
+        document.write('<script src="../Build/Cesium/Cesium.js"><\/script>');
       } else {
-        window.CESIUM_BASE_URL = "../Source/";
-        document.write('<script type="module" src="./SpecList.js"><\/script>');
+        document.write(
+          '<script src="../Build/CesiumUnminified/Cesium.js"><\/script>'
+        );
       }
     </script>
+    <script type="module" src="../Build/Specs/spec-main.js"></script>
+    <script src="./jasmine/boot1.js"></script>
+    <script type="module" src="../Build/Specs/SpecList.js"></script>
   </body>
 </html>
diff --git a/Specs/karma-main.js b/Specs/karma-main.js
index 1bd0672e51f1..c2bcf46fd4d7 100644
--- a/Specs/karma-main.js
+++ b/Specs/karma-main.js
@@ -18,7 +18,7 @@ if (__karma__.config.args) {
 if (release) {
   window.CESIUM_BASE_URL = "base/Build/Cesium";
 } else {
-  window.CESIUM_BASE_URL = "base/Source";
+  window.CESIUM_BASE_URL = "base/Build/CesiumUnminified";
 }
 
 jasmine.DEFAULT_TIMEOUT_INTERVAL = 30000;
diff --git a/Specs/karma.conf.cjs b/Specs/karma.conf.cjs
index 442f35b6f821..f73e240cfdc6 100644
--- a/Specs/karma.conf.cjs
+++ b/Specs/karma.conf.cjs
@@ -27,22 +27,20 @@ module.exports = function (config) {
 
     // list of files / patterns to load in the browser
     files: [
-      { pattern: "Specs/karma-main.js", included: true, type: "module" },
-      { pattern: "Source/**", included: false, type: "module" },
-      { pattern: "Specs/*.js", included: true, type: "module" },
-      { pattern: "Specs/Core/**", included: true, type: "module" },
       { pattern: "Specs/Data/**", included: false },
-      { pattern: "Specs/DataSources/**", included: true, type: "module" },
-      { pattern: "Specs/Renderer/**", included: true, type: "module" },
-      { pattern: "Specs/Scene/**", included: true, type: "module" },
-      { pattern: "Specs/ThirdParty/**", included: true, type: "module" },
-      { pattern: "Specs/Widgets/**", included: true, type: "module" },
-      { pattern: "Specs/TestWorkers/**", included: false },
+      { pattern: "Specs/TestWorkers/**/*.wasm", included: false },
+      { pattern: "Build/CesiumUnminified/Cesium.js", included: true },
+      { pattern: "Build/CesiumUnminified/Cesium.js.map", included: false },
+      { pattern: "Build/CesiumUnminified/**", included: false },
+      { pattern: "Build/Specs/karma-main.js", included: true, type: "module" },
+      { pattern: "Build/Specs/TestWorkers/**", included: false },
+      { pattern: "Build/Specs/SpecList.js", included: true, type: "module" },
     ],
 
     proxies: {
       "/Data": "/base/Specs/Data",
       "/Specs/TestWorkers": "/base/Specs/TestWorkers",
+      "/Build/Specs/TestWorkers": "/base/Build/Specs/TestWorkers",
     },
 
     // list of files to exclude
@@ -50,7 +48,9 @@ module.exports = function (config) {
 
     // preprocess matching files before serving them to the browser
     // available preprocessors: https://npmjs.org/browse/keyword/karma-preprocessor
-    preprocessors: {},
+    preprocessors: {
+      "**/*.js": ["sourcemap"],
+    },
 
     // test results reporter to use
     // possible values: 'dots', 'progress'
diff --git a/Specs/spec-main.js b/Specs/spec-main.js
index 07ecd439391a..20bc9d0e8950 100644
--- a/Specs/spec-main.js
+++ b/Specs/spec-main.js
@@ -6,7 +6,6 @@ const queryString = queryToObject(window.location.search.substring(1));
 
 let webglValidation = false;
 let webglStub = false;
-const built = window.location.search.indexOf("built") !== -1;
 const release = window.location.search.indexOf("release") !== -1;
 const categoryString = queryString.category;
 const excludeCategoryString = queryString.not;
@@ -19,14 +18,10 @@ if (defined(queryString.webglStub)) {
   webglStub = true;
 }
 
-if (built) {
-  if (release) {
-    window.CESIUM_BASE_URL = "../Build/Cesium";
-  } else {
-    window.CESIUM_BASE_URL = "../Build/CesiumUnminified";
-  }
+if (release) {
+  window.CESIUM_BASE_URL = "../Build/Cesium";
 } else {
-  window.CESIUM_BASE_URL = "../Source/";
+  window.CESIUM_BASE_URL = "../Build/CesiumUnminified";
 }
 
 jasmine.DEFAULT_TIMEOUT_INTERVAL = 30000;
diff --git a/Specs/test.cjs b/Specs/test.cjs
new file mode 100644
index 000000000000..66c61107c618
--- /dev/null
+++ b/Specs/test.cjs
@@ -0,0 +1,24 @@
+/*eslint-env node*/
+"use strict";
+
+// NodeJS smoke screen test
+
+const assert = require("node:assert");
+const {
+  CesiumTerrainProvider,
+  Cartographic,
+  sampleTerrain,
+} = require("cesium");
+
+const provider = new CesiumTerrainProvider({
+  url: "https://s3.amazonaws.com/cesiumjs/smallTerrain",
+});
+sampleTerrain(provider, 11, [
+  Cartographic.fromDegrees(86.925145, 27.988257),
+  Cartographic.fromDegrees(87.0, 28.0),
+]).then((results) => {
+  assert(results[0].height > 5000);
+  assert(results[0].height < 10000);
+  assert(results[1].height > 5000);
+  assert(results[1].height < 10000);
+});
diff --git a/gulpfile.cjs b/gulpfile.cjs
index 198b164e0ed6..049c1a0bef5e 100644
--- a/gulpfile.cjs
+++ b/gulpfile.cjs
@@ -2,6 +2,7 @@
 "use strict";
 
 const fs = require("fs");
+const { readFile } = require("fs/promises");
 const path = require("path");
 const os = require("os");
 const child_process = require("child_process");
@@ -12,7 +13,6 @@ const request = require("request");
 
 const globby = require("globby");
 const gulpTap = require("gulp-tap");
-const gulpTerser = require("gulp-terser");
 const open = require("open");
 const rimraf = require("rimraf");
 const glslStripComments = require("glsl-strip-comments");
@@ -29,14 +29,15 @@ const Karma = require("karma");
 const yargs = require("yargs");
 const AWS = require("aws-sdk");
 const mime = require("mime");
+const cleanCSS = require("gulp-clean-css");
+const typescript = require("typescript");
+const esbuild = require("esbuild");
+const istanbul = require("istanbul-lib-instrument");
 const rollup = require("rollup");
 const rollupPluginStripPragma = require("rollup-plugin-strip-pragma");
-const rollupPluginExternalGlobals = require("rollup-plugin-external-globals");
 const rollupPluginTerser = require("rollup-plugin-terser");
 const rollupCommonjs = require("@rollup/plugin-commonjs");
 const rollupResolve = require("@rollup/plugin-node-resolve").default;
-const cleanCSS = require("gulp-clean-css");
-const typescript = require("typescript");
 
 const packageJson = require("./package.json");
 let version = packageJson.version;
@@ -53,28 +54,12 @@ const travisDeployUrl =
 const taskName = process.argv[2];
 const noDevelopmentGallery =
   taskName === "release" || taskName === "makeZipFile";
-const minifyShaders =
-  taskName === "minify" ||
-  taskName === "minifyRelease" ||
-  taskName === "release" ||
-  taskName === "makeZipFile" ||
-  taskName === "buildApps";
-
 const verbose = yargs.argv.verbose;
 
 let concurrency = yargs.argv.concurrency;
 if (!concurrency) {
   concurrency = os.cpus().length;
 }
-
-// Work-around until all third party libraries use npm
-const filesToLeaveInThirdParty = [
-  "!Source/ThirdParty/Workers/basis_transcoder.js",
-  "!Source/ThirdParty/basis_transcoder.wasm",
-  "!Source/ThirdParty/google-earth-dbroot-parser.js",
-  "!Source/ThirdParty/knockout*.js",
-];
-
 const sourceFiles = [
   "Source/**/*.js",
   "!Source/*.js",
@@ -86,20 +71,6 @@ const sourceFiles = [
   "!Source/ThirdParty/_*",
 ];
 
-const watchedFiles = [
-  "Source/**/*.js",
-  "!Source/Cesium.js",
-  "!Source/Build/**",
-  "!Source/Shaders/**/*.js",
-  "Source/Shaders/**/*.glsl",
-  "!Source/ThirdParty/Shaders/*.js",
-  "Source/ThirdParty/Shaders/*.glsl",
-  "!Source/Workers/**",
-  "Source/Workers/cesiumWorkerBootstrapper.js",
-  "Source/Workers/transferTypedArrayTest.js",
-  "!Specs/SpecList.js",
-];
-
 const filesToClean = [
   "Source/Cesium.js",
   "Source/Shaders/**/*.js",
@@ -107,6 +78,7 @@ const filesToClean = [
   "!Source/Workers/cesiumWorkerBootstrapper.js",
   "!Source/Workers/transferTypedArrayTest.js",
   "Source/ThirdParty/Shaders/*.js",
+  "Source/**/*.d.ts",
   "Specs/SpecList.js",
   "Specs/jasmine/**",
   "Apps/Sandcastle/jsHintOptions.js",
@@ -116,30 +88,17 @@ const filesToClean = [
   "cesium-*.tgz",
 ];
 
-const filesToConvertES6 = [
-  "Source/**/*.js",
-  "Specs/**/*.js",
-  "!Source/ThirdParty/**",
-  "!Source/Cesium.js",
-  "!Source/copyrightHeader.js",
-  "!Source/Shaders/**",
-  "!Source/Workers/cesiumWorkerBootstrapper.js",
-  "!Source/Workers/transferTypedArrayTest.js",
-  "!Specs/karma-main.js",
-  "!Specs/karma.conf.cjs",
-  "!Specs/spec-main.js",
-  "!Specs/SpecList.js",
-  "!Specs/TestWorkers/**",
+const workerSourceFiles = ["Source/WorkersES6/**"];
+const watchedSpecFiles = [
+  "Specs/**/*Spec.js",
+  "Specs/*.js",
+  "Specs/TestWorkers/*.js",
+];
+const cssFiles = "Source/**/*.css";
+const shaderFiles = [
+  "Source/Shaders/**/*.glsl",
+  "Source/ThirdParty/Shaders/*.glsl",
 ];
-
-function rollupWarning(message) {
-  // Ignore eval warnings in third-party code we don't have control over
-  if (message.code === "EVAL" && /protobufjs/.test(message.loc.file)) {
-    return;
-  }
-
-  console.log(message);
-}
 
 let copyrightHeader = fs.readFileSync(
   path.join("Source", "copyrightHeader.js"),
@@ -147,81 +106,286 @@ let copyrightHeader = fs.readFileSync(
 );
 copyrightHeader = copyrightHeader.replace("${version}", version);
 
-function createWorkers() {
-  rimraf.sync("Build/createWorkers");
+function escapeCharacters(token) {
+  return token.replace(/[\-\[\]\/\{\}\(\)\*\+\?\.\\\^\$\|]/g, "\\$&");
+}
 
-  globby
-    .sync([
-      "Source/Workers/**",
-      "!Source/Workers/cesiumWorkerBootstrapper.js",
-      "!Source/Workers/transferTypedArrayTest.js",
-    ])
-    .forEach(function (file) {
-      rimraf.sync(file);
-    });
+function constructRegex(pragma, exclusive) {
+  const prefix = exclusive ? "exclude" : "include";
+  pragma = escapeCharacters(pragma);
 
-  const workers = globby.sync(["Source/WorkersES6/**"]);
+  const s =
+    `[\\t ]*\\/\\/>>\\s?${prefix}Start\\s?\\(\\s?(["'])${pragma}\\1\\s?,\\s?pragmas\\.${pragma}\\s?\\)\\s?;?` +
+    // multiline code block
+    `[\\s\\S]*?` +
+    // end comment
+    `[\\t ]*\\/\\/>>\\s?${prefix}End\\s?\\(\\s?(["'])${pragma}\\2\\s?\\)\\s?;?\\s?[\\t ]*\\n?`;
 
-  return rollup
-    .rollup({
-      input: workers,
-      onwarn: rollupWarning,
-    })
-    .then(function (bundle) {
-      return bundle.write({
-        dir: "Build/createWorkers",
-        banner:
-          "/* This file is automatically rebuilt by the Cesium build process. */",
-        format: "amd",
-      });
-    })
-    .then(function () {
-      return streamToPromise(
-        gulp.src("Build/createWorkers/**").pipe(gulp.dest("Source/Workers"))
-      );
-    })
-    .then(function () {
-      rimraf.sync("Build/createWorkers");
+  return new RegExp(s, "gm");
+}
+
+const pragmas = {
+  debug: false,
+};
+const stripPragmaPlugin = {
+  name: "strip-pragmas",
+  setup: (build) => {
+    build.onLoad({ filter: /\.js$/ }, async (args) => {
+      let source = await readFile(args.path, "utf8");
+
+      try {
+        for (const key in pragmas) {
+          if (pragmas.hasOwnProperty(key)) {
+            source = source.replace(constructRegex(key, pragmas[key]), "");
+          }
+        }
+
+        return { contents: source };
+      } catch (e) {
+        return {
+          errors: {
+            text: e.message,
+          },
+        };
+      }
     });
+  },
+};
+
+// Print an esbuild warning
+function printBuildWarning({ location, text }) {
+  const { column, file, line, lineText, suggestion } = location;
+
+  let message = `\n
+  > ${file}:${line}:${column}: warning: ${text}
+  ${lineText}
+  `;
+
+  if (suggestion && suggestion !== "") {
+    message += `\n${suggestion}`;
+  }
+
+  console.log(message);
 }
 
-async function buildThirdParty() {
-  rimraf.sync("Build/createWorkers");
-  globby.sync(filesToLeaveInThirdParty).forEach(function (file) {
-    rimraf.sync(file);
+// Ignore `eval` warnings in third-party code we don't have control over
+function handleBuildWarnings(result) {
+  for (const warning of result.warnings) {
+    if (
+      !warning.location.file.includes("protobufjs.js") &&
+      !warning.location.file.includes("Build/Cesium")
+    ) {
+      printBuildWarning(warning);
+    }
+  }
+}
+
+const esbuildBaseConfig = () => {
+  return {
+    target: "es2020",
+    legalComments: "inline",
+    banner: {
+      js: copyrightHeader,
+    },
+  };
+};
+
+async function buildCesiumJs(options) {
+  const css = globby.sync(cssFiles);
+
+  const buildConfig = esbuildBaseConfig();
+  buildConfig.entryPoints = ["Source/Cesium.js"];
+  buildConfig.bundle = true;
+  buildConfig.minify = options.minify;
+  buildConfig.sourcemap = options.sourcemap;
+  buildConfig.external = ["https", "http", "url", "zlib"];
+  buildConfig.plugins = options.removePragmas ? [stripPragmaPlugin] : undefined;
+  buildConfig.incremental = options.incremental;
+  buildConfig.logLevel = "error"; // print errors immediately, and collect warnings so we can filter out known ones
+
+  // Build ESM
+  const result = await esbuild.build({
+    ...buildConfig,
+    format: "esm",
+    outfile: path.join(options.path, "index.js"),
   });
 
-  const workers = globby.sync(["ThirdParty/npm/**"]);
+  handleBuildWarnings(result);
 
-  return rollup
-    .rollup({
-      input: workers,
-      plugins: [rollupResolve(), rollupCommonjs()],
-      onwarn: rollupWarning,
-    })
-    .then(function (bundle) {
-      return bundle.write({
-        dir: "Build/createThirdPartyNpm",
-        banner:
-          "/* This file is automatically rebuilt by the Cesium build process. */",
-        format: "es",
-      });
-    })
-    .then(function () {
-      return streamToPromise(
-        gulp
-          .src("Build/createThirdPartyNpm/**")
-          .pipe(gulp.dest("Source/ThirdParty"))
+  const results = [result];
+
+  // Copy and minify CSS and third party
+  const config = esbuildBaseConfig();
+  config.entryPoints = [
+    "Source/ThirdParty/google-earth-dbroot-parser.js",
+    ...css, // Load and optionally minify css
+  ];
+  config.loader = {
+    ".gif": "text",
+    ".png": "text",
+  };
+  config.minify = options.minify;
+  config.sourcemap = options.sourcemap;
+  config.outdir = options.path;
+  await esbuild.build(config);
+
+  // Build IIFE
+  if (options.iife) {
+    const result = await esbuild.build({
+      ...buildConfig,
+      format: "iife",
+      globalName: "Cesium",
+      outfile: path.join(options.path, "Cesium.js"),
+    });
+
+    handleBuildWarnings(result);
+
+    results.push(result);
+  }
+
+  if (options.node) {
+    const result = await esbuild.build({
+      ...buildConfig,
+      format: "cjs",
+      platform: "node",
+      sourcemap: false,
+      define: {
+        // TransformStream is a browser-only implementation depended on by zip.js
+        TransformStream: "null",
+      },
+      outfile: path.join(options.path, "index.cjs"),
+    });
+
+    handleBuildWarnings(result);
+    results.push(result);
+  }
+
+  return results;
+}
+
+function rollupWarning(message) {
+  // Ignore eval warnings in third-party code we don't have control over
+  if (message.code === "EVAL" && /protobufjs/.test(message.loc.file)) {
+    return;
+  }
+
+  console.log(message);
+}
+
+/**
+ * Bundles the workers and outputs the result to the specified directory
+ * @param {Object} options
+ * @param {boolean} [options.minify=false] true if the worker output should be minified
+ * @param {boolean} [options.removePragmas=false] true if debug pragma should be removed
+ * @param {boolean} [options.sourcemap=false] true if an external sourcemap should be generated
+ * @param {String} options.path output directory
+ */
+async function buildWorkers(options) {
+  // Copy existing workers
+  const workers = globby.sync([
+    "Source/Workers/**",
+    "Source/ThirdParty/Workers/**",
+  ]);
+
+  const config = esbuildBaseConfig();
+  config.entryPoints = workers;
+  config.outdir = options.path;
+  config.outbase = "Source"; // Maintain existing file paths
+  config.minify = options.minify;
+  await esbuild.build(config);
+
+  // Use rollup to build the workers:
+  // 1) They can be built as AMD style modules
+  // 2) They can be built using code-splitting, resulting in smaller modules
+  return globby(["Source/WorkersES6/*.js"]).then(function (files) {
+    const plugins = [rollupResolve(), rollupCommonjs()];
+
+    if (options.removePragmas) {
+      plugins.push(
+        rollupPluginStripPragma({
+          pragmas: ["debug"],
+        })
       );
-    })
-    .then(function () {
-      rimraf.sync("Build/createThirdPartyNpm");
+    }
+
+    if (options.minify) {
+      plugins.push(rollupPluginTerser.terser());
+    }
+
+    return rollup
+      .rollup({
+        input: files,
+        plugins: plugins,
+        onwarn: rollupWarning,
+      })
+      .then(function (bundle) {
+        return bundle.write({
+          dir: path.join(options.path, "Workers"),
+          format: "amd",
+          // Rollup cannot generate a sourcemap when pragmas are removed
+          sourcemap: options.sourcemap && !options.removePragmas,
+          banner: copyrightHeader,
+        });
+      });
+  });
+}
+
+const externalResolvePlugin = {
+  name: "external-cesium",
+  setup: (build) => {
+    build.onResolve({ filter: new RegExp(`Cesium\.js$`) }, () => {
+      return {
+        path: "Cesium",
+        namespace: "external-cesium",
+      };
     });
+
+    build.onLoad(
+      {
+        filter: new RegExp(`^Cesium$`),
+        namespace: "external-cesium",
+      },
+      () => {
+        const contents = `module.exports = Cesium`;
+        return {
+          contents,
+        };
+      }
+    );
+  },
+};
+
+async function buildSpecs(options) {
+  options = options || {};
+
+  const results = await esbuild.build({
+    entryPoints: [
+      "Specs/spec-main.js",
+      "Specs/SpecList.js",
+      "Specs/karma-main.js",
+    ],
+    bundle: true,
+    format: "esm",
+    sourcemap: true,
+    target: "es2020",
+    outdir: path.join("Build", "Specs"),
+    plugins: [externalResolvePlugin],
+    incremental: options.incremental,
+  });
+
+  return results;
 }
 
-gulp.task("build", async function () {
+async function build(options) {
+  options = options || {};
   mkdirp.sync("Build");
 
+  const outputDirectory = path.join(
+    "Build",
+    `Cesium${!options.minify ? "Unminified" : ""}`
+  );
+  rimraf.sync(outputDirectory);
+
   fs.writeFileSync(
     "Build/package.json",
     JSON.stringify({
@@ -230,18 +394,154 @@ gulp.task("build", async function () {
     "utf8"
   );
 
-  await buildThirdParty();
-  glslToJavaScript(minifyShaders, "Build/minifyShaders.state");
+  glslToJavaScript(options.minify, "Build/minifyShaders.state");
   createCesiumJs();
   createSpecList();
   createJsHintOptions();
-  return Promise.join(createWorkers(), createGalleryList());
-});
+  return Promise.join(
+    buildCesiumJs({
+      minify: options.minify,
+      iife: true,
+      sourcemap: options.sourcemap,
+      removePragmas: options.removePragmas,
+      path: outputDirectory,
+      node: options.node,
+    }),
+    buildWorkers({
+      minify: options.minify,
+      sourcemap: options.sourcemap,
+      path: outputDirectory,
+      removePragmas: options.removePragmas,
+    }),
+    createGalleryList(),
+    buildSpecs()
+  ).then(() => {
+    return copyAssets({
+      outputDirectory: outputDirectory,
+    });
+  });
+}
 
-gulp.task("build-watch", function () {
-  return gulp.watch(watchedFiles, gulp.series("build"));
+gulp.task("build", function () {
+  const argv = yargs.argv;
+  const minify = argv.minify ? argv.minify : false;
+  const removePragmas = argv.pragmas ? argv.pragmas : false;
+  const sourcemap = argv.sourcemap ? argv.sourcemap : true;
+  const node = argv.node ? argv.node : true;
+
+  return build({
+    minify: minify,
+    removePragmas: removePragmas,
+    sourcemap: sourcemap,
+    node: node,
+  });
 });
 
+gulp.task(
+  "build-watch",
+  gulp.series("build", async function () {
+    const argv = yargs.argv;
+    const minify = argv.minify ? argv.minify : false;
+    const removePragmas = argv.pragmas ? argv.pragmas : false;
+    const sourcemap = argv.sourcemap ? argv.sourcemap : true;
+
+    const outputDirectory = path.join(
+      "Build",
+      `Cesium${!minify ? "Unminified" : ""}`
+    );
+
+    let [esmResult, iifeResult, cjsResult] = await buildCesiumJs({
+      minify: minify,
+      path: outputDirectory,
+      removePragmas: removePragmas,
+      sourcemap: sourcemap,
+      incremental: true,
+    });
+
+    let specResult = await buildSpecs({
+      incremental: true,
+    });
+
+    await buildWorkers({
+      minify: minify,
+      path: outputDirectory,
+      removePragmas: removePragmas,
+      sourcemap: sourcemap,
+    });
+
+    gulp.watch(shaderFiles, async () => {
+      glslToJavaScript(minify, "Build/minifyShaders.state");
+      esmResult = await esmResult.rebuild();
+
+      if (iifeResult) {
+        iifeResult = await iifeResult.rebuild();
+      }
+
+      if (cjsResult) {
+        cjsResult = await cjsResult.rebuild();
+      }
+    });
+
+    gulp.watch(sourceFiles, async () => {
+      esmResult = await esmResult.rebuild();
+
+      if (iifeResult) {
+        iifeResult = await iifeResult.rebuild();
+      }
+
+      if (cjsResult) {
+        cjsResult = await cjsResult.rebuild();
+      }
+    });
+
+    gulp.watch(
+      watchedSpecFiles,
+      {
+        events: ["add", "unlink"],
+      },
+      async () => {
+        createSpecList();
+        specResult = await specResult.rebuild();
+      }
+    );
+
+    gulp.watch(
+      watchedSpecFiles,
+      {
+        events: ["change"],
+      },
+      async () => {
+        specResult = await specResult.rebuild();
+      }
+    );
+
+    gulp.watch(workerSourceFiles, () => {
+      return buildWorkers({
+        minify: minify,
+        path: outputDirectory,
+        removePragmas: removePragmas,
+        sourcemap: sourcemap,
+      });
+    });
+
+    process.on("SIGINT", () => {
+      // Free up resources
+      esmResult.rebuild.dispose();
+
+      if (iifeResult) {
+        iifeResult.rebuild.dispose();
+      }
+
+      if (cjsResult) {
+        cjsResult.rebuild.dispose();
+      }
+
+      specResult.rebuild.dispose();
+      process.exit(0);
+    });
+  })
+);
+
 gulp.task("build-ts", function () {
   createTypeScriptDefinitions();
   return Promise.resolve();
@@ -251,51 +551,6 @@ gulp.task("buildApps", function () {
   return Promise.join(buildCesiumViewer(), buildSandcastle());
 });
 
-gulp.task("build-specs", function buildSpecs() {
-  const externalCesium = rollupPluginExternalGlobals({
-    "../Source/Cesium.js": "Cesium",
-    "../../Source/Cesium.js": "Cesium",
-    "../../../Source/Cesium.js": "Cesium",
-    "../../../../Source/Cesium.js": "Cesium",
-  });
-
-  const removePragmas = rollupPluginStripPragma({
-    pragmas: ["debug"],
-  });
-
-  const promise = Promise.join(
-    rollup
-      .rollup({
-        input: "Specs/SpecList.js",
-        plugins: [externalCesium],
-        onwarn: rollupWarning,
-      })
-      .then(function (bundle) {
-        return bundle.write({
-          file: "Build/Specs/Specs.js",
-          format: "iife",
-        });
-      })
-      .then(function () {
-        return rollup
-          .rollup({
-            input: "Specs/karma-main.js",
-            plugins: [removePragmas, externalCesium],
-            onwarn: rollupWarning,
-          })
-          .then(function (bundle) {
-            return bundle.write({
-              file: "Build/Specs/karma-main.js",
-              name: "karmaMain",
-              format: "iife",
-            });
-          });
-      })
-  );
-
-  return promise;
-});
-
 gulp.task("build-third-party", function () {
   return generateThirdParty();
 });
@@ -351,28 +606,7 @@ function cloc() {
 
 gulp.task("cloc", gulp.series("clean", cloc));
 
-function combine() {
-  const outputDirectory = path.join("Build", "CesiumUnminified");
-  return combineJavaScript({
-    removePragmas: false,
-    minify: false,
-    outputDirectory: outputDirectory,
-  });
-}
-
-gulp.task("combine", gulp.series("build", combine));
-gulp.task("default", gulp.series("combine"));
-
-function combineRelease() {
-  const outputDirectory = path.join("Build", "CesiumUnminified");
-  return combineJavaScript({
-    removePragmas: true,
-    minify: false,
-    outputDirectory: outputDirectory,
-  });
-}
-
-gulp.task("combineRelease", gulp.series("build", combineRelease));
+gulp.task("default", gulp.series("build"));
 
 gulp.task("prepare", function () {
   // Copy Draco3D files from node_modules into Source
@@ -450,10 +684,21 @@ gulp.task("generateDocumentation-watch", function () {
 gulp.task(
   "release",
   gulp.series(
-    "build",
+    function () {
+      return build({
+        minify: false,
+        removePragmas: false,
+        node: true,
+      });
+    },
+    function () {
+      return build({
+        minify: true,
+        removePragmas: true,
+        node: true,
+      });
+    },
     "build-ts",
-    combine,
-    minifyRelease,
     generateDocumentation
   )
 );
@@ -469,21 +714,16 @@ gulp.task(
     delete packageJson.scripts.prepare;
 
     // Remove build and transform tasks since they do not function as intended from within the release zip
-    delete packageJson.scripts.convertToModules;
     delete packageJson.scripts.build;
     delete packageJson.scripts["build-watch"];
     delete packageJson.scripts["build-ts"];
     delete packageJson.scripts["build-third-party"];
-    delete packageJson.scripts.buildApps;
+    delete packageJson.scripts["buildApps"];
     delete packageJson.scripts.clean;
     delete packageJson.scripts.cloc;
-    delete packageJson.scripts.combine;
-    delete packageJson.scripts.combineRelease;
-    delete packageJson.scripts.generateDocumentation;
+    delete packageJson.scripts["generateDocumentation"];
     delete packageJson.scripts["generateDocumentation-watch"];
-    delete packageJson.scripts.makeZipFile;
-    delete packageJson.scripts.minify;
-    delete packageJson.scripts.minifyRelease;
+    delete packageJson.scripts["makeZipFile"];
     delete packageJson.scripts.release;
     delete packageJson.scripts.prettier;
 
@@ -507,6 +747,7 @@ gulp.task(
         "Build/CesiumUnminified/**",
         "Build/Documentation/**",
         "Build/package.json",
+        // TODO: Include built specs? Shouldn't be too big
       ],
       {
         base: ".",
@@ -564,27 +805,6 @@ gulp.task(
   })
 );
 
-gulp.task(
-  "minify",
-  gulp.series("build", function () {
-    return combineJavaScript({
-      removePragmas: false,
-      minify: true,
-      outputDirectory: path.join("Build", "Cesium"),
-    });
-  })
-);
-
-function minifyRelease() {
-  return combineJavaScript({
-    removePragmas: true,
-    minify: true,
-    outputDirectory: path.join("Build", "Cesium"),
-  });
-}
-
-gulp.task("minifyRelease", gulp.series("build", minifyRelease));
-
 function isTravisPullRequest() {
   return (
     process.env.TRAVIS_PULL_REQUEST !== undefined &&
@@ -963,7 +1183,7 @@ function setStatus(state, targetUrl, description, context) {
   });
 }
 
-gulp.task("coverage", function (done) {
+gulp.task("coverage", async function () {
   const argv = yargs.argv;
   const webglStub = argv.webglStub ? argv.webglStub : false;
   const suppressPassed = argv.suppressPassed ? argv.suppressPassed : false;
@@ -975,69 +1195,134 @@ gulp.task("coverage", function (done) {
     browsers = argv.browsers.split(",");
   }
 
-  const karmaConfig = Karma.config.parseConfig(karmaConfigFile, {
-    port: 9876,
-    browsers: browsers,
-    specReporter: {
-      suppressErrorSummary: false,
-      suppressFailed: false,
-      suppressPassed: suppressPassed,
-      suppressSkipped: true,
-    },
-    preprocessors: {
-      "Source/Core/**/*.js": ["karma-coverage-istanbul-instrumenter"],
-      "Source/DataSources/**/*.js": ["karma-coverage-istanbul-instrumenter"],
-      "Source/Renderer/**/*.js": ["karma-coverage-istanbul-instrumenter"],
-      "Source/Scene/**/*.js": ["karma-coverage-istanbul-instrumenter"],
-      "Source/Shaders/**/*.js": ["karma-coverage-istanbul-instrumenter"],
-      "Source/Widgets/**/*.js": ["karma-coverage-istanbul-instrumenter"],
-      "Source/Workers/**/*.js": ["karma-coverage-istanbul-instrumenter"],
-    },
-    coverageIstanbulInstrumenter: {
-      esModules: true,
-    },
-    reporters: ["spec", "coverage"],
-    coverageReporter: {
-      dir: "Build/Coverage",
-      subdir: function (browserName) {
-        folders.push(browserName);
-        return browserName;
-      },
-      includeAllSources: true,
-    },
-    client: {
-      captureConsole: verbose,
-      args: [
-        undefined,
-        undefined,
-        undefined,
-        undefined,
-        undefined,
-        webglStub,
-        undefined,
-      ],
-    },
+  const instrumenter = new istanbul.createInstrumenter({
+    esModules: true,
   });
 
-  const karma = new Karma.Server(karmaConfig, function doneCallback(exitCode) {
-    let html = "<!doctype html><html><body><ul>";
-    folders.forEach(function (folder) {
-      html += `<li><a href="${encodeURIComponent(
-        folder
-      )}/index.html">${folder}</a></li>`;
-    });
-    html += "</ul></body></html>";
-    fs.writeFileSync("Build/Coverage/index.html", html);
+  const instrumentPlugin = {
+    name: "instrument",
+    setup: (build) => {
+      const readFile = Promise.promisify(fs.readFile);
+      build.onLoad(
+        {
+          filter: /Source\/(Core|DataSources|Renderer|Scene|Widgets)(\/\w+)+\.js$/,
+        },
+        async (args) => {
+          const source = await readFile(args.path, "utf8");
+
+          try {
+            const generatedCode = instrumenter.instrumentSync(
+              source,
+              args.path
+            );
+
+            return { contents: generatedCode };
+          } catch (e) {
+            return {
+              errors: {
+                text: e.message,
+              },
+            };
+          }
+        }
+      );
+    },
+  };
 
-    if (!process.env.TRAVIS) {
-      folders.forEach(function (dir) {
-        open(`Build/Coverage/${dir}/index.html`);
-      });
-    }
-    return done(failTaskOnError ? exitCode : undefined);
+  const outputDirectory = path.join("Build", "Instrumented");
+
+  const result = await esbuild.build({
+    entryPoints: ["Source/Cesium.js"],
+    bundle: true,
+    sourcemap: true,
+    format: "iife",
+    globalName: "Cesium",
+    target: "es2020",
+    external: ["https", "http", "url", "zlib"],
+    outfile: path.join(outputDirectory, "Cesium.js"),
+    plugins: [instrumentPlugin],
+    logLevel: "error", // print errors immediately, and collect warnings so we can filter out known ones
   });
 
-  karma.start();
+  handleBuildWarnings(result);
+
+  return new Promise((resolve, reject) => {
+    const karma = new Karma.Server(
+      {
+        configFile: karmaConfigFile,
+        browsers: browsers,
+        specReporter: {
+          suppressErrorSummary: false,
+          suppressFailed: false,
+          suppressPassed: suppressPassed,
+          suppressSkipped: true,
+        },
+        files: [
+          { pattern: "Specs/Data/**", included: false },
+          { pattern: "Specs/TestWorkers/**/*.wasm", included: false },
+          { pattern: "Build/Instrumented/Cesium.js", included: true },
+          { pattern: "Build/Instrumented/Cesium.js.map", included: false },
+          { pattern: "Build/CesiumUnminified/**", included: false },
+          {
+            pattern: "Build/Specs/karma-main.js",
+            included: true,
+            type: "module",
+          },
+          {
+            pattern: "Build/Specs/SpecList.js",
+            included: true,
+            type: "module",
+          },
+          { pattern: "Specs/TestWorkers/**", included: false },
+        ],
+        reporters: ["spec", "coverage"],
+        coverageReporter: {
+          dir: "Build/Coverage",
+          subdir: function (browserName) {
+            folders.push(browserName);
+            return browserName;
+          },
+          includeAllSources: true,
+        },
+        client: {
+          captureConsole: false,
+          args: [
+            undefined,
+            undefined,
+            undefined,
+            undefined,
+            undefined,
+            webglStub,
+            undefined,
+          ],
+        },
+      },
+      function (e) {
+        let html = "<!doctype html><html><body><ul>";
+        folders.forEach(function (folder) {
+          html += `<li><a href="${encodeURIComponent(
+            folder
+          )}/index.html">${folder}</a></li>`;
+        });
+        html += "</ul></body></html>";
+        fs.writeFileSync("Build/Coverage/index.html", html);
+
+        if (!process.env.TRAVIS) {
+          folders.forEach(function (dir) {
+            open(`Build/Coverage/${dir}/index.html`);
+          });
+        }
+
+        if (failTaskOnError && e) {
+          reject(e);
+          return;
+        }
+
+        resolve();
+      }
+    );
+    karma.start();
+  });
 });
 
 gulp.task("test", function (done) {
@@ -1060,28 +1345,27 @@ gulp.task("test", function (done) {
   }
 
   let files = [
-    { pattern: "Specs/karma-main.js", included: true, type: "module" },
-    { pattern: "Source/**", included: false, type: "module" },
-    { pattern: "Specs/*.js", included: true, type: "module" },
-    { pattern: "Specs/Core/**", included: true, type: "module" },
     { pattern: "Specs/Data/**", included: false },
-    { pattern: "Specs/DataSources/**", included: true, type: "module" },
-    { pattern: "Specs/Renderer/**", included: true, type: "module" },
-    { pattern: "Specs/Scene/**", included: true, type: "module" },
-    { pattern: "Specs/ThirdParty/**", included: true, type: "module" },
-    { pattern: "Specs/Widgets/**", included: true, type: "module" },
+    { pattern: "Specs/TestWorkers/**/*.wasm", included: false },
+    { pattern: "Build/CesiumUnminified/Cesium.js", included: true },
+    { pattern: "Build/CesiumUnminified/Cesium.js.map", included: false },
+    { pattern: "Build/CesiumUnminified/**", included: false },
+    { pattern: "Build/Specs/karma-main.js", included: true, type: "module" },
+    { pattern: "Build/Specs/SpecList.js", included: true, type: "module" },
     { pattern: "Specs/TestWorkers/**", included: false },
   ];
 
   if (release) {
     files = [
       { pattern: "Specs/Data/**", included: false },
+      { pattern: "Specs/TestWorkers/**/*.wasm", included: false },
       { pattern: "Specs/ThirdParty/**", included: false, type: "module" },
-      { pattern: "Specs/TestWorkers/**", included: false },
       { pattern: "Build/Cesium/Cesium.js", included: true },
+      { pattern: "Build/Cesium/Cesium.js.map", included: false },
       { pattern: "Build/Cesium/**", included: false },
       { pattern: "Build/Specs/karma-main.js", included: true },
-      { pattern: "Build/Specs/Specs.js", included: true },
+      { pattern: "Build/Specs/SpecList.js", included: true, type: "module" },
+      { pattern: "Specs/TestWorkers/**", included: false },
     ];
   }
 
@@ -1119,310 +1403,28 @@ gulp.task("test", function (done) {
   karma.start();
 });
 
-gulp.task("convertToModules", function () {
-  const requiresRegex = /([\s\S]*?(define|defineSuite|require)\((?:{[\s\S]*}, )?\[)([\S\s]*?)]([\s\S]*?function\s*)\(([\S\s]*?)\) {([\s\S]*)/;
-  const noModulesRegex = /([\s\S]*?(define|defineSuite|require)\((?:{[\s\S]*}, )?\[?)([\S\s]*?)]?([\s\S]*?function\s*)\(([\S\s]*?)\) {([\s\S]*)/;
-  const splitRegex = /,\s*/;
-
-  const fsReadFile = Promise.promisify(fs.readFile);
-  const fsWriteFile = Promise.promisify(fs.writeFile);
-
-  const files = globby.sync(filesToConvertES6);
-
-  return Promise.map(files, function (file) {
-    return fsReadFile(file).then(function (contents) {
-      contents = contents.toString();
-      if (contents.startsWith("import")) {
-        return;
-      }
-
-      let result = requiresRegex.exec(contents);
-
-      if (result === null) {
-        result = noModulesRegex.exec(contents);
-        if (result === null) {
-          return;
-        }
-      }
-
-      const names = result[3].split(splitRegex);
-      if (names.length === 1 && names[0].trim() === "") {
-        names.length = 0;
-      }
-
-      for (let i = 0; i < names.length; ++i) {
-        if (names[i].indexOf("//") >= 0 || names[i].indexOf("/*") >= 0) {
-          console.log(
-            `${file} contains comments in the require list.  Skipping so nothing gets broken.`
-          );
-          return;
-        }
-      }
-
-      const identifiers = result[5].split(splitRegex);
-      if (identifiers.length === 1 && identifiers[0].trim() === "") {
-        identifiers.length = 0;
-      }
-
-      for (let i = 0; i < identifiers.length; ++i) {
-        if (
-          identifiers[i].indexOf("//") >= 0 ||
-          identifiers[i].indexOf("/*") >= 0
-        ) {
-          console.log(
-            `${file} contains comments in the require list.  Skipping so nothing gets broken.`
-          );
-          return;
-        }
-      }
-
-      const requires = [];
-
-      for (let i = 0; i < names.length && i < identifiers.length; ++i) {
-        requires.push({
-          name: names[i].trim(),
-          identifier: identifiers[i].trim(),
-        });
-      }
-
-      // Convert back to separate lists for the names and identifiers, and add
-      // any additional names or identifiers that don't have a corresponding pair.
-      const sortedNames = requires.map(function (item) {
-        return `${item.name.slice(0, -1)}.js'`;
-      });
-      for (let i = sortedNames.length; i < names.length; ++i) {
-        sortedNames.push(names[i].trim());
-      }
-
-      const sortedIdentifiers = requires.map(function (item) {
-        return item.identifier;
-      });
-      for (let i = sortedIdentifiers.length; i < identifiers.length; ++i) {
-        sortedIdentifiers.push(identifiers[i].trim());
-      }
-
-      contents = "";
-      if (sortedNames.length > 0) {
-        for (let q = 0; q < sortedNames.length; q++) {
-          let modulePath = sortedNames[q];
-          if (file.startsWith("Specs")) {
-            modulePath = modulePath.substring(1, modulePath.length - 1);
-            const sourceDir = path.dirname(file);
-
-            if (modulePath.startsWith("Specs") || modulePath.startsWith(".")) {
-              let importPath = modulePath;
-              if (modulePath.startsWith("Specs")) {
-                importPath = path.relative(sourceDir, modulePath);
-                if (importPath[0] !== ".") {
-                  importPath = `./${importPath}`;
-                }
-              }
-              modulePath = `'${importPath}'`;
-              contents += `import ${sortedIdentifiers[q]} from ${modulePath};${os.EOL}`;
-            } else {
-              modulePath =
-                `'${path.relative(sourceDir, "Source")}/Cesium.js` + `'`;
-              if (sortedIdentifiers[q] === "CesiumMath") {
-                contents += `import { Math as CesiumMath } from ${modulePath};${os.EOL}`;
-              } else {
-                contents += `import { ${sortedIdentifiers[q]} } from ${modulePath};${os.EOL}`;
-              }
-            }
-          } else {
-            contents += `import ${sortedIdentifiers[q]} from ${modulePath};${os.EOL}`;
-          }
-        }
-      }
-
-      let code;
-      const codeAndReturn = result[6];
-      if (file.endsWith("Spec.js")) {
-        const indi = codeAndReturn.lastIndexOf("});");
-        code = codeAndReturn.slice(0, indi);
-        code = code.trim().replace(`'use strict';${os.EOL}`, "");
-        contents += code + os.EOL;
-      } else {
-        const returnIndex = codeAndReturn.lastIndexOf("return");
-
-        code = codeAndReturn.slice(0, returnIndex);
-        code = code.trim().replace(`'use strict';${os.EOL}`, "");
-        contents += code + os.EOL;
-
-        const returnStatement = codeAndReturn.slice(returnIndex);
-        contents += `${returnStatement
-          .split(";")[0]
-          .replace("return ", "export default ")};${os.EOL}`;
-      }
-
-      return fsWriteFile(file, contents);
-    });
-  });
-});
-
-function combineCesium(debug, minify, combineOutput) {
-  const plugins = [];
-
-  if (!debug) {
-    plugins.push(
-      rollupPluginStripPragma({
-        pragmas: ["debug"],
-      })
-    );
-  }
-  if (minify) {
-    plugins.push(rollupPluginTerser.terser());
-  }
-
-  return rollup
-    .rollup({
-      input: "Source/Cesium.js",
-      plugins: plugins,
-      onwarn: rollupWarning,
-    })
-    .then(function (bundle) {
-      return bundle.write({
-        format: "umd",
-        name: "Cesium",
-        file: path.join(combineOutput, "Cesium.js"),
-        sourcemap: debug,
-        banner: copyrightHeader,
-      });
-    });
-}
-
-function combineWorkers(debug, minify, combineOutput) {
-  //This is done waterfall style for concurrency reasons.
-  // Copy files that are already minified
-  return globby(["Source/ThirdParty/Workers/draco*.js"])
-    .then(function (files) {
-      const stream = gulp
-        .src(files, { base: "Source" })
-        .pipe(gulp.dest(combineOutput));
-      return streamToPromise(stream);
-    })
-    .then(function () {
-      return globby([
-        "Source/Workers/cesiumWorkerBootstrapper.js",
-        "Source/Workers/transferTypedArrayTest.js",
-        "Source/ThirdParty/Workers/*.js",
-        // Files are already minified, don't optimize
-        "!Source/ThirdParty/Workers/draco*.js",
-      ]);
-    })
-    .then(function (files) {
-      return Promise.map(
-        files,
-        function (file) {
-          return streamToPromise(
-            gulp
-              .src(file)
-              .pipe(gulpTerser())
-              .pipe(
-                gulp.dest(
-                  path.dirname(
-                    path.join(combineOutput, path.relative("Source", file))
-                  )
-                )
-              )
-          );
-        },
-        { concurrency: concurrency }
-      );
-    })
-    .then(function () {
-      return globby(["Source/WorkersES6/*.js"]);
-    })
-    .then(function (files) {
-      const plugins = [];
-
-      if (!debug) {
-        plugins.push(
-          rollupPluginStripPragma({
-            pragmas: ["debug"],
-          })
-        );
-      }
-      if (minify) {
-        plugins.push(rollupPluginTerser.terser());
-      }
-
-      return rollup
-        .rollup({
-          input: files,
-          plugins: plugins,
-          onwarn: rollupWarning,
-        })
-        .then(function (bundle) {
-          return bundle.write({
-            dir: path.join(combineOutput, "Workers"),
-            format: "amd",
-            sourcemap: debug,
-            banner: copyrightHeader,
-          });
-        });
-    });
-}
-
-function minifyCSS(outputDirectory) {
-  streamToPromise(
-    gulp
-      .src("Source/**/*.css")
-      .pipe(cleanCSS())
-      .pipe(gulp.dest(outputDirectory))
-  );
-}
-
-function minifyModules(outputDirectory) {
-  return streamToPromise(
-    gulp
-      .src("Source/ThirdParty/google-earth-dbroot-parser.js")
-      .pipe(gulpTerser())
-      .pipe(gulp.dest(`${outputDirectory}/ThirdParty/`))
-  );
-}
-
-function combineJavaScript(options) {
-  const minify = options.minify;
+/**
+ * Copies non-js assets to the output directory
+ *
+ * @param {Object} options
+ * @param {String} options.outputDirectory
+ * @returns
+ */
+function copyAssets(options) {
   const outputDirectory = options.outputDirectory;
-  const removePragmas = options.removePragmas;
-
-  const combineOutput = path.join(
-    "Build",
-    "combineOutput",
-    minify ? "minified" : "combined"
-  );
-
-  const promise = Promise.join(
-    combineCesium(!removePragmas, minify, combineOutput),
-    combineWorkers(!removePragmas, minify, combineOutput),
-    minifyModules(outputDirectory)
-  );
-
-  return promise.then(function () {
-    const promises = [];
-
-    //copy to build folder with copyright header added at the top
-    let stream = gulp
-      .src([`${combineOutput}/**`])
-      .pipe(gulp.dest(outputDirectory));
-
-    promises.push(streamToPromise(stream));
-
-    const everythingElse = ["Source/**", "!**/*.js", "!**/*.glsl"];
-    if (minify) {
-      promises.push(minifyCSS(outputDirectory));
-      everythingElse.push("!**/*.css");
-    }
+  const everythingElse = [
+    "Source/**",
+    "!**/*.js",
+    "!**/*.glsl",
+    "!**/*.css",
+    "!**/*.md",
+  ];
 
-    stream = gulp
-      .src(everythingElse, { nodir: true })
-      .pipe(gulp.dest(outputDirectory));
-    promises.push(streamToPromise(stream));
+  const stream = gulp
+    .src(everythingElse, { nodir: true })
+    .pipe(gulp.dest(outputDirectory));
 
-    return Promise.all(promises).then(function () {
-      rimraf.sync(combineOutput);
-    });
-  });
+  return streamToPromise(stream);
 }
 
 function glslToJavaScript(minify, minifyStateFilePath) {
@@ -1445,10 +1447,7 @@ function glslToJavaScript(minify, minifyStateFilePath) {
   const builtinConstants = [];
   const builtinStructs = [];
 
-  const glslFiles = globby.sync([
-    "Source/Shaders/**/*.glsl",
-    "Source/ThirdParty/Shaders/*.glsl",
-  ]);
+  const glslFiles = globby.sync(shaderFiles);
   glslFiles.forEach(function (glslFile) {
     glslFile = path.normalize(glslFile);
     const baseName = path.basename(glslFile, ".glsl");
@@ -1717,10 +1716,10 @@ ${source}
 }
 
 function createSpecList() {
-  const specFiles = globby.sync(["Specs/**/*Spec.js"]);
+  const files = globby.sync(["Specs/**/*Spec.js"]);
 
   let contents = "";
-  specFiles.forEach(function (file) {
+  files.forEach(function (file) {
     contents += `import './${filePathToModuleId(file).replace(
       "Specs/",
       ""
@@ -1982,14 +1981,14 @@ function buildSandcastle() {
       "!Apps/Sandcastle/images/**",
       "!Apps/Sandcastle/gallery/**.jpg",
     ])
-    // Remove dev-only ES6 module loading for unbuilt Cesium
+    // Remove swap out ESM modules for the IIFE build
     .pipe(
       gulpReplace(
         '    <script type="module" src="../load-cesium-es6.js"></script>',
-        ""
+        '    <script src="../../../Build/CesiumUnminified/Cesium.js"></script>\n' +
+          '    <script>window.CESIUM_BASE_URL = "../../../Build/CesiumUnminified/";</script>";'
       )
     )
-    .pipe(gulpReplace("nomodule", ""))
     // Fix relative paths for new location
     .pipe(gulpReplace("../../../Build", "../../.."))
     .pipe(gulpReplace("../../Source", "../../../Source"))
@@ -2010,94 +2009,84 @@ function buildSandcastle() {
     .pipe(
       gulpReplace(
         '    <script type="module" src="load-cesium-es6.js"></script>',
-        ""
+        '    <script src="../../Build/CesiumUnminified/Cesium.js"></script>\n' +
+          '    <script>window.CESIUM_BASE_URL = "../../Build/CesiumUnminified/";</script>";'
       )
     )
-    .pipe(gulpReplace("nomodule", ""))
     .pipe(gulpReplace("../../Build", "../.."))
     .pipe(gulp.dest("Build/Apps/Sandcastle"));
 
   return streamToPromise(mergeStream(appStream, imageStream, standaloneStream));
 }
 
-function buildCesiumViewer() {
+async function buildCesiumViewer() {
   const cesiumViewerOutputDirectory = "Build/Apps/CesiumViewer";
   mkdirp.sync(cesiumViewerOutputDirectory);
 
-  let promise = Promise.join(
-    rollup
-      .rollup({
-        input: "Apps/CesiumViewer/CesiumViewer.js",
-        treeshake: {
-          moduleSideEffects: false,
-        },
-        plugins: [
-          rollupPluginStripPragma({
-            pragmas: ["debug"],
-          }),
-          rollupPluginTerser.terser(),
-        ],
-        onwarn: rollupWarning,
-      })
-      .then(function (bundle) {
-        return bundle.write({
-          file: "Build/Apps/CesiumViewer/CesiumViewer.js",
-          format: "iife",
-        });
-      })
-  );
+  const config = esbuildBaseConfig();
+  config.entryPoints = [
+    "Apps/CesiumViewer/CesiumViewer.js",
+    "Apps/CesiumViewer/CesiumViewer.css",
+  ];
+  config.bundle = true; // Tree-shaking is enabled automatically
+  config.minify = true;
+  config.loader = {
+    ".gif": "text",
+    ".png": "text",
+  };
+  config.format = "iife";
+  config.inject = ["Apps/CesiumViewer/index.js"];
+  config.external = ["https", "http", "zlib"];
+  config.outdir = cesiumViewerOutputDirectory;
+  config.outbase = "Apps/CesiumViewer";
+  config.logLevel = "error"; // print errors immediately, and collect warnings so we can filter out known ones
+  const result = await esbuild.build(config);
+  handleBuildWarnings(result);
+
+  await esbuild.build({
+    entryPoints: ["Source/Widgets/InfoBox/InfoBoxDescription.css"],
+    minify: true,
+    bundle: true,
+    loader: {
+      ".gif": "text",
+      ".png": "text",
+    },
+    outdir: cesiumViewerOutputDirectory,
+    outbase: "Source",
+  });
 
-  promise = promise.then(function () {
-    const stream = mergeStream(
-      gulp
-        .src("Build/Apps/CesiumViewer/CesiumViewer.js")
-        .pipe(gulpInsert.prepend(copyrightHeader))
-        .pipe(gulpReplace("../../Source", "."))
-        .pipe(gulp.dest(cesiumViewerOutputDirectory)),
-
-      gulp
-        .src("Apps/CesiumViewer/CesiumViewer.css")
-        .pipe(cleanCSS())
-        .pipe(gulpReplace("../../Source", "."))
-        .pipe(gulp.dest(cesiumViewerOutputDirectory)),
-
-      gulp
-        .src("Apps/CesiumViewer/index.html")
-        .pipe(gulpReplace('type="module"', ""))
-        .pipe(gulp.dest(cesiumViewerOutputDirectory)),
-
-      gulp.src([
-        "Apps/CesiumViewer/**",
-        "!Apps/CesiumViewer/index.html",
-        "!Apps/CesiumViewer/**/*.js",
-        "!Apps/CesiumViewer/**/*.css",
-      ]),
-
-      gulp.src(
-        [
-          "Build/Cesium/Assets/**",
-          "Build/Cesium/Workers/**",
-          "Build/Cesium/ThirdParty/**",
-          "Build/Cesium/Widgets/**",
-          "!Build/Cesium/Widgets/**/*.css",
-        ],
-        {
-          base: "Build/Cesium",
-          nodir: true,
-        }
-      ),
+  await buildWorkers({
+    minify: true,
+    removePragmas: true,
+    path: cesiumViewerOutputDirectory,
+  });
 
-      gulp.src(["Build/Cesium/Widgets/InfoBox/InfoBoxDescription.css"], {
-        base: "Build/Cesium",
-      }),
+  const stream = mergeStream(
+    gulp.src([
+      "Apps/CesiumViewer/**",
+      "!Apps/CesiumViewer/Images",
+      "!Apps/CesiumViewer/**/*.js",
+      "!Apps/CesiumViewer/**/*.css",
+    ]),
 
-      gulp.src(["web.config"])
-    );
+    gulp.src(
+      [
+        "Build/Cesium/Assets/**",
+        "Build/Cesium/Workers/**",
+        "Build/Cesium/ThirdParty/**",
+        "Build/Cesium/Widgets/**",
+        "!Build/Cesium/Widgets/**/*.css",
+      ],
+      {
+        base: "Build/Cesium",
+        nodir: true,
+      }
+    ),
 
-    return streamToPromise(stream.pipe(gulp.dest(cesiumViewerOutputDirectory)));
-  });
+    gulp.src(["web.config"])
+  );
 
-  return promise;
+  return streamToPromise(stream.pipe(gulp.dest(cesiumViewerOutputDirectory)));
 }
 
 function filePathToModuleId(moduleId) {
diff --git a/index.cjs b/index.cjs
index 01788d101da0..a0dde5dee5d2 100644
--- a/index.cjs
+++ b/index.cjs
@@ -6,8 +6,11 @@ const path = require("path");
 // If in 'production' mode, use the combined/minified/optimized version of Cesium
 if (process.env.NODE_ENV === "production") {
   // eslint-disable-next-line global-require
-  module.exports = require(path.join(__dirname, "Build/Cesium/Cesium"));
+  module.exports = require(path.join(__dirname, "Build/Cesium/index.cjs"));
   return;
 }
 
-module.exports = require(path.join(__dirname, "Build/CesiumUnminified/Cesium"));
+module.exports = require(path.join(
+  __dirname,
+  "Build/CesiumUnminified/index.cjs"
+));
diff --git a/package.json b/package.json
index 59fd0120ad03..407841c0423a 100644
--- a/package.json
+++ b/package.json
@@ -53,6 +53,7 @@
     "dompurify": "^2.2.2",
     "draco3d": "^1.5.1",
     "earcut": "^2.2.2",
+    "esbuild": "^0.14.38",
     "eslint": "^8.19.0",
     "eslint-config-prettier": "^8.3.0",
     "eslint-config-cesium": "^9.0.0",
@@ -69,16 +70,15 @@
     "gulp-rename": "^2.0.0",
     "gulp-replace": "^1.1.3",
     "gulp-tap": "^2.0.0",
-    "gulp-terser": "^2.0.1",
     "gulp-zip": "^5.1.0",
     "husky": "^7.0.2",
+    "istanbul-lib-instrument": "^5.2.0",
     "jasmine-core": "^4.0.1",
     "jsdoc": "^3.6.7",
     "jsep": "^0.3.1",
     "karma": "^6.3.20",
     "karma-chrome-launcher": "^3.1.0",
     "karma-coverage": "^2.0.3",
-    "karma-coverage-istanbul-instrumenter": "^1.0.3",
     "karma-detect-browsers": "^2.3.3",
     "karma-edge-launcher": "^0.4.2",
     "karma-firefox-launcher": "^2.1.1",
@@ -86,6 +86,7 @@
     "karma-jasmine": "^4.0.1",
     "karma-longest-reporter": "^1.1.0",
     "karma-safari-launcher": "^1.0.0",
+    "karma-sourcemap-loader": "^0.3.8",
     "karma-spec-reporter": "^0.0.32",
     "kdbush": "^3.0.0",
     "ktx-parse": "^0.2.1",
@@ -106,7 +107,6 @@
     "request": "^2.79.0",
     "rimraf": "^3.0.2",
     "rollup": "^2.52.2",
-    "rollup-plugin-external-globals": "^0.6.1",
     "rollup-plugin-strip-pragma": "^1.0.0",
     "rollup-plugin-terser": "^7.0.2",
     "stream-to-promise": "^3.0.0",
@@ -119,28 +119,25 @@
   },
   "scripts": {
     "prepare": "gulp prepare && husky install",
-    "convertToModules": "gulp convertToModules",
     "start": "node server.cjs",
     "startPublic": "node server.cjs --public",
     "build": "gulp build",
+    "minify": "gulp build --minify",
+    "minifyRelease": "gulp build --minify --removePragmas",
+    "combineRelease": "gulp build --removePragmas",
     "build-watch": "gulp build-watch",
     "build-ts": "gulp build-ts",
     "build-third-party": "gulp build-third-party",
     "buildApps": "gulp buildApps",
     "clean": "gulp clean",
     "cloc": "gulp cloc",
-    "combine": "gulp combine",
-    "combineRelease": "gulp combineRelease",
     "coverage": "gulp coverage",
     "generateDocumentation": "gulp generateDocumentation",
     "generateDocumentation-watch": "gulp generateDocumentation-watch",
     "eslint": "eslint \"./**/*.js\" \"./**/*.cjs\" \"./**/*.html\" --cache --quiet",
     "makeZipFile": "gulp makeZipFile",
     "markdownlint": "markdownlint \"*.md\" \"Documentation/**/*.md\" --ignore CHANGES.md --ignore README.md --ignore LICENSE.md",
-    "minify": "gulp minify",
-    "minifyRelease": "gulp minifyRelease",
     "release": "gulp release",
-    "build-specs": "gulp build-specs",
     "test": "gulp test",
     "test-all": "gulp test --all",
     "test-webgl": "gulp test --include WebGL",
@@ -154,6 +151,9 @@
     "prettier": "prettier --write \"**/*\"",
     "prettier-check": "prettier --check \"**/*\""
   },
+  "engines": {
+    "node": ">=14.0.0"
+  },
   "lint-staged": {
     "*.{js,cjs,html}": [
       "eslint --cache --quiet",