Do you know how good your tests are? Mutation testing can tell you. Unlike test coverage metrics (which only tell us how much of your application was executed, not whether the tests were any use), mutation testing lets us say something concrete about the quality of your test suite. Mutation testing has been around for years, but it's only recently that performance tools (such as PI Test for Java) have become available. In this session, Seb will look at the motivation and technology behind mutation testing and see some examples in action.