Bill Venners: When do you stop writing tests? You say in Refactoring, "There's a point of diminishing returns with testing, and there's a danger that by writing too many tests you become discouraged and end up not writing any. You should concentrate on where the risk is." How do you know where the risk is? 

Martin Fowler: Ask yourself which bits of the program would you be scared to change? One test I've come up with since the Refactoring book is asking if there is any line of code that you could comment out and the tests wouldn't fail? If so, you are either missing a test or you've got an unnecessary line of code. Similarly, take any Boolean expression. Could you just reverse it? What test would fail? If there's not a test failing, then, you've obviously got some more tests to write or some code to remove.