This chapter describes the theory of planned behavior (TPB), a prominent reasoned action model, its conceptual foundation, its intellectual history, and the research it has generated. From its roots in propositional control and expectancy theory, the TPB emerged as a major framework for understanding, predicting, and changing human social behavior. According to the theory, intention is the immediate antecedent of behavior and is itself a function of attitude toward the behavior, subjective norm, and perceived behavioral control; and these determinants follow, respectively, from beliefs about the behavior’s likely consequences, about normative expectations of important others, and about the presence of factors that control behavioral performance. Empirical support for the theory comes from a host of correlational studies demonstrating its ability to predict intentions and behavior as well as from interventions showing that changes in behavioral, normative, and control beliefs can produce changes in intentions, and that these changes in intentions are reflected in subsequent behavior. The chapter also considers the TPB’s reasoned action approach in the context of recent work on automatic, nonconscious processes in human social behavior. It is argued that insight into automaticity can complement the understanding of behavior provided by a reasoned action approach.