I review the different theories of sex differences in cognition and social behavior as well as gender development in this chapter. I begin by describing biological theories that focus on genes, hormones, and brain structure. Then I describe how evolutionary theory has been applied to sex differences by emphasizing the distinct challenges that men and women face in ensuring the survival of their genes. Each of these theories is increasingly interactionistic, recognizing that predisposing factors require environmental input to have their effects. I discuss several environmental theories, beginning with social learning theory, which emphasizes modeling and reinforcement of sex-typed behavior; turn to gender-socialization agents such as parents, peers, and the media; and then describe social role theory, which has its roots in the division of labor that a culture assigns to women and men. In contrast to environmental theories, cognitive development theory emphasizes the stages of gender identity acquisition through which children proceed. Gender schema theory combines principles of social learning theory and cognitive development theory to describe the process by which children acquire and enact sex-typed behavior. The chapter concludes with a more proximal theory of gender behavior, emphasizing the role that situational variables have on the display of sex differences.