Abstract. Modeling and documentation are two essential ingredients for the engineering discipline of software development. During the last twenty years a wide variety of description and modeling techniques as well as document formats has been proposed. However, often these are not integrated into a coherent methodology with well-defined dependencies between the models and documentations. This hampers focused software development as well as the provision of powerful tool-support. In this paper we present the main issues and outline solutions in the direction of a unified, formal basis for software and system modeling.