This chapter integrates research in several academic disciplines on the lay theories that people rely on when making sense of their social environments. First, drawing on philosophy of science, we establish several criteria for a theory and distinguish types or levels of theories. Then we review work in philosophy on folk psychology, in developmental psychology research on the child’s theory of mind, and in social psychology research on implicit theories, causal schemata, and scripts. Against the foil of work in other disciplines, lay theory proposals by social psychologists take on a new perspective. We conclude by considering how the theory construct serves in accounting for individual differences and cultural differences in social cognition.