Knowledge-based modelling consists of qualitative models which are implemented in a hybrid, rule- and frame-oriented programming language. Qualitative models achieve flexible and detailed description of both the simulation entities and relationships. This paper presents a simulation expert system which is based on a multi-level representation scheme of causal diagrams. It offers a qualitative “presimulation”, that is conclusions about the sensitivity of the elements, the restrictions and the possible behaviour of the model. It enables explanation of the various implicit structural and dynamic relationships and the user to be guided to efficient quantitative simulation.