Within the framework of so-called second generation expert systems  kwledge modeling is one of the most important aspects. On the one hand, kwledge acquisition is longer seen as a kwledge transfer process, rather it is w considered as model construction process which is typically a cyclic and error prone process. On the other hand, the distinction between kwledge and symbol level descriptions  resulted in various proposals for conceptual kwledge models describing kwledge in an implementation independent way. One of the most prominent examples of such a conceptual model is the KADS model of expertise which is characterized by its clear distinction of different kw ledge types and by the usage of specific modeling primitives to describe these different kwledge types . The semi- formal KADS expertise model entails all the advantages and disadvantages which have been identified for semi-formal system models e.g. in the software engineering community.