Avoid testing implementation details, test behaviors

Ian Cooper writes an excellent piece on avoiding test implementation details, concentrating on the Red, Green, Refactor cycle, and what actually requires testing. He goes on to explain how new classes are not triggers for new tests, but rather new requirements. Overall a well written article that distills some of the values Kent Beck's book: Test Driven Development. If you find yourself struggling with TDD, I'm sure this will clear things right up.