A design model for parametric systems is presented based on the example of vehicle design. The formal construction of the model is equivalent to an “expert system using conventional means”.The knowledge base comprises algorithmic and non-algorithmic sections. It can store knowledge from different sources (designer's know-how, mathematical/physical simulation etc.). A graphic interactive user interface allowes to set up the enquiry, which is taken by the “solution engine” to compile a reply at the user interface using an optimum target-oriented search technique on the knowledge base. In addition to the user interface, also a graphic interactive consultation and explanation component is available to support the user in setting up consistent questions and in interpreting the results.Practical application of the design model is shown with the aid of a simple example.