Abstract: Model Based Testing in Evolutionary Software Development A. Pretschner and H. Lötzbeyer and J. Philipps The spiraling nature of evolutionary software development processes produces executable parts of the system at the end of each loop. We argue that these parts should consist not only of programming language code, but of executable graphical system models. As a main benefit of the use of more abstract, yet formal, modeling languages, we present a method for model based test sequence generation for reactive systems on the grounds of Constraint Logic Programming and its prototypical implementation in the CASE tool AutoFocus.