This paper discusses a model-based approach to testing as a vital part of software development. It argues that an approach using models as central development artifact needs to be added to the portfolio of software engineering techniques, to further increase efficiency and flexibility of the development as well as quality and reusability of results. Then test case modeling is examined in depth and related to an evolutionary approach to model transformation. A number of test patterns is proposed that have proven helpful to the design of testable object-oriented systems. In contrast to other approaches, this approach uses explicit models for test cases instead of trying to derive (many) test cases from a single model.