This chapter served to shed some light into the hidden interior of JUnit. It was not our goal to examine every detail of this open test framework, but instead to give the interested developer some useful hints for his or her entry into source text reading. In addition, a certain basic knowledge of the static structure and the dynamics of the most important objects is useful for those who want to evaluate the usability of test frameworks for specific test projects and scenarios. In fact, project-specific expansions of JUnit are often required, whereas complex expansions and reorganizations of the framework are involved in a project only in rare cases.