This chapter reviews the five propositions that constitute organismic integration theory (OIT). The five propositions address (1) the process of internalization; (2) the distinctions between intrinsic motivation, the different types of extrinsic motivation, and amotivation, and how they relate to each other; (3) the conditions that facilitate internalization of values and behaviors; (4) how behavior that is regulated through more internalized forms of regulation relates to better functioning; and (5) how internalization relates to psychological health and well-being. Also examined are aspects of the theory that have received less attention or that represent emerging trends, such as the contribution of the constructs of integrated regulation and amotivation to SDT’s model of motivation. The chapter argues that OIT provides a framework to explain how society and social contexts affect people’s motivation, as well as what people do in actively adapting to their social worlds. It also sees potential for OIT to expand from the examination of individual human functioning and wellness to understanding how people’s prosocial behaviors and willingness to contribute to others develops and how societies can support this.