The aim in the research described here is to construct a model for design processes for technical systems that should help to uncover the principles behind design phenomena and structure the relevant design knowledge. The approach taken in this research can be best described as trying to regard design as an (applied) exact science, rather than as a craft or an art (Alberts et al. 1991a). Up to this point there is no such thing as a complete design theory in any of the design domains. So for the time being experience-based design knowledge will keep on playing a central role. However, we propose to structure this experiental knowledge in terms of the underlying technical and physical principles.