diff --git a/README.md b/README.md
index bb7b878..8b876cc 100644
--- a/README.md
+++ b/README.md
@@ -382,6 +382,20 @@ var failing = expect.createSpy()
 
 Restores a spy originally created with `expect.spyOn()`.
 
+## Extending expect
+
+To create new expectations, add functions to `expect.fn` and call `expect.assert`.
+
+```js
+expect.fn.toBeAColor = function () {
+  expect.assert(this.actual.match(/^#[a-fA-F0-9]{6}$/),
+    'expected %s to be an HTML color',
+    this.actual)
+}
+
+expect('#ff00ff').toBeAColor()
+```
+
 ## Issues
 
 Please file issues on the [issue tracker on GitHub](https://github.com/mjackson/expect/issues).
diff --git a/modules/__tests__/extension-test.js b/modules/__tests__/extension-test.js
new file mode 100644
index 0000000..3819fe1
--- /dev/null
+++ b/modules/__tests__/extension-test.js
@@ -0,0 +1,23 @@
+/*eslint-env mocha */
+import expect from '../index'
+
+describe('expect.fn', function () {
+  beforeEach(function () {
+    expect.spyOn(expect, 'assert')
+    expect.fn.toBeAColor = function () {
+      expect.assert(this.actual.match(/^#[a-fA-F0-9]{6}$/),
+        'expected %s to be an HTML color',
+        this.actual)
+    }
+  })
+
+  afterEach(function () {
+    expect.assert.restore()
+    delete expect.fn.toBeAColor
+  })
+
+  it('works', function () {
+    expect('#ff00ff').toBeAColor()
+    expect(expect.assert).toHaveBeenCalled()
+  })
+})
diff --git a/modules/index.js b/modules/index.js
index b9bd720..0f48336 100644
--- a/modules/index.js
+++ b/modules/index.js
@@ -1,5 +1,6 @@
 import Expectation from './Expectation'
 import { createSpy, spyOn, isSpy, restoreSpies } from './SpyUtils'
+import invariant from './invariant'
 
 function expect(actual) {
   return new Expectation(actual)
@@ -9,5 +10,7 @@ expect.createSpy = createSpy
 expect.spyOn = spyOn
 expect.isSpy = isSpy
 expect.restoreSpies = restoreSpies
+expect.fn = Expectation.prototype
+expect.assert = invariant
 
 export default expect