Writing tests for the functionality of your site and modules. Includes:

• Advantages (and caveats) of using Simpletest
• When to use Simpletest
• What is test-driven development (TDD)?
• How Simpletest works
• Setup and running a test
• Anatomy of a .test file
• Writing your first test
• Running your first test
• Simpletests and forms
• The Simpletest API and further reading
• Submitting a patch to Drupal.org
• Summary

Code can be found here.