diff --git a/doc/README.md b/doc/README.md
index d74f2d32594..8cffeaaff07 100644
--- a/doc/README.md
+++ b/doc/README.md
@@ -40,4 +40,5 @@ Additional features:
 * [Pagination support](result_pager.md)
 * [Authentication & Security](security.md)
 * [Request any Route](request_any_route.md)
-* [Customize `php-github-api` and testing](customize.md)
+* [Customize `php-github-api`](customize.md)
+* [Running and writing tests](testing.md)
diff --git a/doc/customize.md b/doc/customize.md
index 8d7536e0e91..f25f4e40dc8 100644
--- a/doc/customize.md
+++ b/doc/customize.md
@@ -1,4 +1,4 @@
-## Customize `php-github-api` and testing
+## Customize `php-github-api`
 [Back to the navigation](README.md)
 
 
@@ -41,11 +41,3 @@ $httpBuilder->addPlugin(new CustomUserAgentPlugin());
 
 $client = new Github\Client($httpBuilder);
 ```
-
-### Run Test Suite
-
-The code is unit tested, there are also some functional tests. To run tests on your machine, from a CLI, run
-
-```bash
-$ phpunit
-```
diff --git a/doc/testing.md b/doc/testing.md
new file mode 100644
index 00000000000..ec2effb3b5c
--- /dev/null
+++ b/doc/testing.md
@@ -0,0 +1,73 @@
+## Running and writing tests
+[Back to the navigation](README.md)
+
+
+### Run Test Suite
+
+The code is unit tested, there are also some functional tests. To run tests on 
+your machine, from a CLI, run
+
+```bash
+$ composer update
+$ phpunit
+```
+
+### Write tests
+
+It is always great if someone wants to contribute and extend the functionality of 
+the API client. But all new features must be properly tested. To test a new API 
+function, one should test its communication with the HTTP client. The code should 
+never make an actual call to Github. Testing could easily be done with mocking. 
+
+If you want to write test for the function that shows you comments to a gist. 
+
+```php
+class Comments extends AbstractApi
+{
+    // ...
+    public function show($gist, $comment)
+    {
+        return $this->get('/gists/'.rawurlencode($gist).'/comments/'.rawurlencode($comment));
+    }
+}
+```
+
+The test will look like this: 
+
+```php
+use Github\Tests\Api\TestCase;
+
+class CommentsTest extends TestCase
+{
+    // ...
+
+    /**
+     * @test
+     */
+    public function shouldShowGistComment()
+    {
+        // Create a variable with the "Server response".
+        $expectedValue = array('comment1');
+
+        // Get the API mock (see "getApiClass" below).
+        $api = $this->getApiMock();
+        
+        $api->expects($this->once())                    // Expect one call
+            ->method('get')                             // A GET request
+            ->with('/gists/123/comments/456')           // URI should be "/gists/123/comments/456"
+            ->will($this->returnValue($expectedValue)); // Should return the "Server response"
+
+        // Call Comments::show
+        $result = $api->show(123, 456);
+        
+        // Verify that the result is the "Server response" as we expect. 
+        $this->assertEquals($expectedValue, $result);
+    }
+    
+    protected function getApiClass()
+    {
+        // Tell the "getAPIMock" what class to mock. 
+        return \Github\Api\Gist\Comments::class;
+    }
+}
+```