With little or no formal teaching, human beings develop the capacity to deploy psychological concepts in predicting and explaining the actions and mental states of other members of the species. What is the basis for this capacity? Many philosophers and psychologists argue that normal adult human beings possess a primitive or "folk" psychological theory. Recently, however, this theory has come under challenge from the simulation alternative. This alternative view says that human beings are able to predict and explain each others' actions by using the resources of their own minds to simulate the psychological etiology of the actions of others. The 13 essays in this volume present the foundations of theory of mind debate, and are accompanied by an extensive introduction. Accompanion volume, Mental Simulation: Evaluations and Applications, edited by Tony Stone and Martin Davies, Blackwells (1995) contains thirteen further essays, most of which appear there for the first time. Taken together, the two volumes offer a richly woven fabric of philosophical and psychological theory, which promises to yield real insight into the nature of mental lives.