In the last years the role of software in vehicles has grown dramatically. Body electronic features and driver support functions turn out to be crucial for the success of modern vehicles. Automotive software will become even more complex and expensive in future. lt is planned to extend established functions and to implement new functionality which will use the formerly isolated ECUs (electronic control units) as an integrated network. These functions will be realized as cooperating distributed real-time tasks. Thus, on the one hand system complexity will grow, on the other hand short time-to-market cycles and a considerable number of variants needs a well-structured development process supporting the reuse of components at all phases. We aim for a seamless development process for automotive functions with structured methods supported by case tools. The paper presents a major part of the STEP-X project (Structured Development Process by the example of X-By-Wire-Application in the automotive), namely a seamless, model based software development process in automotive engineering. Our process is model based and supported by a tool chain. The tool DOORS is used for requirements management and engineering whereas the CASE tool Artisan RtS based on the Unified Modeling Language (UML) and the CASE tool Ascet SD are used for specification and design purposes. Each of these tools has its particular strength in a certain design phase. We propose designing rules and modeling guidelines for the development of state based behavior which conforms to seamless model transformation in our tool chain. The rules are checked by an embedded rulechecker. Additionally we illustrate our approach in a case study on a subsystem of the Volkswagen car electronics. The case study is characterized by state-oriented and concurrent behavior as well as time and value-discrete information processing.