Earnest Engineer

When you are inside the code for an application, often this question doesn't get pondered. It seems like an obvious question, "What to test?" "Duh, everything!" or "What I just implemented."  But really, testing everything is often out of the question for a reasonably complex application, The permutations of states and settings grows quickly. If you have 4 settings which can each be one of two values, that's 16 (2 x 2 x 2 x 2) different combinations to be tested. Add another setting of only 2 choices and you have 32 combinations. If each setting is one of three values, there are 81 (3 x 3 x 3 x 3) combinations.  I think you can see the problem. If you have a good unit testing framework that will let you automate generation of numerous test cases, that can help a lot. But you still need to consider the question and the answer and decide if you are testing the right parts of the application , whether it's unit tests, manual tests, system test