This paper describes desiderata for an environment to implement the model-based codesign methodology. A brief summary of model-based codesign is given, followed by a discussion of related work. The services and capabilities required of a design environment are given within the concept of a CAD framework. The design flow that realizes model-base codesign is presented. Attention is given to the required activities at each step and to the flow of design information between design steps. The design flow described here focuses on real-time embedded systems. It covers engineering activities from requirements documentation to the physical realization of the design. Emphasis is placed on modeling and simulation to support automated design reasoning. Considerations related to tool support and tool integration are also given.