This chapter is about how to tell whether your program is correct. It discusses systematic testing, and how this can be automated, e.g. using a unit testing framework such as JUnit. It covers what tests you should have, and when you should write them, mentioning Test-Driven Development, an approach to programming in which you write the tests before you write the code that is to be tested. Finally it discusses property-based testing, commonly used in Haskell. In this approach you specify and test something about the relationship between the inputs and outputs of your program, and that relationship is tested on many randomly-chosen input examples.