Mutation testing is a technique that measures the quality of a test suite and helps you write more robust code. This is done by making small changes (mutations) to our code, which will break our application, and ensuring that there is at least one test that no longer passes. This talk introduces the concept of mutation testing. We show that by using mutation testing we can find bugs in code which has a 100% code coverage. We will look at a few open source projects and use the padraic/humbug package to analyze their Mutation Score Indicator. Lastly we look at some of the disadvantages of mutation testing, namely performance and false positives and how to overcome them.