In this interdisciplinary discussion on mental models researchers from various areas in cognitive science tackle the following questions: What is a mental model; what are the prospects and limitations in applying the mental model notion in cognitive science; and how can the ideas on the nature of mental models and their mode of operation be empirically substantiated? The primary goal of the research group was to work out a definition of mental models that embraces the overall use of this construct in cognitive science as well as the more specific conceptions used in particular research domains such as cognitive linguistics. Theoretical claims about the properties of mental models were discussed and their tenability evaluated against the empirical evidence. According to the discussions in the research group the volume is divided into three parts: Fundamental aspects of mental models are presented in the first part, the following part contains contributions to the function of mental models in discourse processing, problems of mental models in reasoning and problem solving are outlined in the last part.