Up until now there has been no agreement on what a mental model of a physical system is and how to infer the mental model a person has. This paper describes research aimed at solving these problems by proposing that a Mental Model is a dynamic representation created in WM by combining information stored in LTM (the Conceptual Model of the system) and characteristics extracted from the environment. Three experiments tested hypotheses derived from this proposal. Implications for research on Mental Model are discussed. Key words: Mental Model, Representation, Working Memory, Long-term Memory.