diff --git a/src/__tests__/reduceBy.js b/src/__tests__/reduceBy.js
new file mode 100644
index 0000000..cdce61a
--- /dev/null
+++ b/src/__tests__/reduceBy.js
@@ -0,0 +1,42 @@
+/* eslint-disable no-nested-ternary */
+import reduceBy from '../reduceBy';
+
+describe('Core.reduceBy', () => {
+  test('splits the list into groups according to the grouping function', () => {
+    const grade = score =>
+      score < 65
+        ? 'F'
+        : score < 70 ? 'D' : score < 80 ? 'C' : score < 90 ? 'B' : 'A';
+
+    const students = [
+      { name: 'Abby', score: 84 },
+      { name: 'Brad', score: 73 },
+      { name: 'Chris', score: 89 },
+      { name: 'Dianne', score: 99 },
+      { name: 'Eddy', score: 58 },
+      { name: 'Fred', score: 67 },
+      { name: 'Gillian', score: 91 },
+      { name: 'Hannah', score: 78 },
+      { name: 'Irene', score: 85 },
+      { name: 'Jack', score: 69 },
+    ];
+
+    const byGrade = student => grade(student.score || 0);
+    const collectNames = (acc, student) => acc.concat(student.name);
+
+    expect(reduceBy(collectNames, [], byGrade, students)).toEqual({
+      A: ['Dianne', 'Gillian'],
+      B: ['Abby', 'Chris', 'Irene'],
+      C: ['Brad', 'Hannah'],
+      D: ['Fred', 'Jack'],
+      F: ['Eddy'],
+    });
+  });
+
+  test('returns an empty object if given an empty array', () => {
+    const byType = obj => obj.type;
+    const sumValues = (acc, obj) => acc + obj.val;
+
+    expect(reduceBy(sumValues, 0, byType, [])).toEqual({});
+  });
+});
diff --git a/src/countBy.js b/src/countBy.js
index f770c15..51597d4 100644
--- a/src/countBy.js
+++ b/src/countBy.js
@@ -1,16 +1,6 @@
-import reduce from './reduce';
-import prop from './prop';
-import assoc from './assoc';
+import reduceBy from './reduceBy';
+import inc from './inc';
 
-const countBy = (fn, list) =>
-  reduce(
-    (acc, item) => {
-      const key = fn(item);
-      const existingKey = prop(key, acc);
-      return assoc(key, existingKey ? existingKey + 1 : 1, acc);
-    },
-    {},
-    list,
-  );
+const countBy = (keyFn, list) => reduceBy(inc, 0, keyFn, list);
 
 export { countBy as default };
diff --git a/src/index.js b/src/index.js
index 364fc00..065c4f2 100644
--- a/src/index.js
+++ b/src/index.js
@@ -45,6 +45,7 @@ import prepend from './prepend';
 import product from './product';
 import prop from './prop';
 import reduce from './reduce';
+import reduceBy from './reduceBy';
 import reduceRight from './reduceRight';
 import reverse from './reverse';
 import slice from './slice';
@@ -102,6 +103,7 @@ export {
   product,
   prop,
   reduce,
+  reduceBy,
   reduceRight,
   reverse,
   slice,
diff --git a/src/reduceBy.js b/src/reduceBy.js
new file mode 100644
index 0000000..d124e7d
--- /dev/null
+++ b/src/reduceBy.js
@@ -0,0 +1,16 @@
+import reduce from './reduce';
+import has from './has';
+import assoc from './assoc';
+
+const reduceBy = (valFn, valAcc, keyFn, list) =>
+  reduce(
+    (acc, item) => {
+      const key = keyFn(item);
+      const val = valFn(has(key, acc) ? acc[key] : valAcc, item);
+      return assoc(key, val, acc);
+    },
+    {},
+    list,
+  );
+
+export { reduceBy as default };