This paper presents a method (CASA-ME) and a tool environment (CASA-TE) supporting the design of computer based systems. The methodology is based on architectural descriptions which not only allow the descriptions of components, connections and composite structures, but also provide a mechanism to formulate architectural related design knowledge. Beside these architectural descriptions CASA-ME also provides a mechanism allowing the formulation and traceability of requirements. Simulation models built with external tools and coordinated by CASA-TE will be used to validate and verify these requirements. After elaborating on the different concepts of CASA-ME a few words will be said about CASA-TE which is under development. Small examples will be used to demonstrate the different concepts of CASA-ME.