Testing is not a stand-alone activity. It has its place with in a software development life cycle model and therefore the life cycle applied will largely determine how testing is organized. There are many different forms of testing. Because several disciplines, often with different interests, are involved in the development life cycle, it is important to clearly understand and define the various test levels and types. This chapter discusses the most commonly applied software development models, test levels and test types. Maintenance can be seen as a specific instance of a development process. The way maintenance influences the test process, levels and types and how testing can be organized is described in the last section of this chapter.