Religion and religiosity are often conceptualized as belief, behavior, and belonging. These three dimensions place religion as a phenomenon for study squarely in the purview of social psychological theories. Moreover, the interdependent relationship between religious (and nonreligious) belief, behavior, and belonging, at macro, meso, and micro levels, is a fruitful area for social psychological research. At the micro level, social psychology illuminates the social construction and sources of specific beliefs, performance and frequency of religious behaviors, and association with a congregation or another type of religious group. At the meso level, theory clarifies the nature and function of shared beliefs, group practices, and cooperation and conflict between religious organizations. At the macro level, social psychological theory informs investigation of systems of belief, socially structured practices, and group attachment. Thus, in this chapter, we highlight research that applies social psychological theory in the investigation of religious (and nonreligious) individuals, the groups to which they belong, and the societies in which they and their (non-)religious groups exist, conflict, cooperate, and change. We also point toward productive areas for new and additional research that will further the social scientific study of religion, social psychological research, and their intersection.