"Unit tests are one of the corner stones of Extreme Programming (XP). But unit tests XP style is a little different. First you should create or download a unit test framework to be able to create automated unit tests suites. Second you should test all classes in the system. Trivial getter and setter methods are usually omitted. And last you should try to create your tests first, before the code."

-http://www.extremeprogramming.org/rules/unittests.html