One of the central tenets of Extreme Programming (XP) is relentless testing throughout the development cycle. In the XP methodology, QA personnel, or "testers," are responsible for interacting with the users to create acceptance tests, while developers are responsible for unit testing of classes, subsystems, and frameworks (components). This chapter discusses a model for collaboration between developers and testers in which testers augment the developers' unit testing activities without additional process overhead.