Nowadays, mental models are seen as crucial in systems design. Research is driven by the assumption that a better insight into a user's cognitive processes when using a system will improve design methods and provide friendly and efficient interfaces. The papers in this volume explore three fundamental issues: understanding the complexity of the intended worksystem, describing it by models and finally building the required powerful and usable system. The papers are an edited selection of those presented at the 8th interdisciplinary workshop on Mental Models and HCI, held in Austria in June 1989. They concentrate primarily on design issues, their theoretical background and the application of the concept of Human-Computer Interaction (HCI). Nevertheless, there are also contributions on theoretical topics and methodological questions.