How good is your test suite? Would it all still pass if the tested code was changed? If so, there may be problems with your code, your tests, or both! Mutation Testing reveals these cases. It makes lots of slightly altered versions of your code, called "mutants". If any mutants let all of the code’s tests pass, that implies certain flaws. This talk will tell you how to use mutation testing, its benefits, drawbacks, inner workings, and history. There will be several examples (in clear pseudocode, or JavaScript), and a list of tools for many popular languages. You will come away equipped with a powerful new technique for making sure your tests are strict and your code is meaningful.