Regular participation in physical activities of sufficient duration and intensity has been consistently associated with adaptive health benefits among people of all ages, including special populations. This chapter provides a review on self-determination theory in the physical activity contexts. First, the chapter presents the benefits of and guidelines on regular physical activity participation. Next, it reviews the effectiveness of self-determination theory in identifying the determinants of physical activity behavior and the processes involved. Specifically, the relationships between psychological need satisfaction, autonomous motivation, and physical activity participation will be examined. Next, the chapter reviews studies related to the process of internalization in physical activity participation and intervention studies in creating an autonomy-supportive environment. It also reviews studies that integrate self-determination theory with other theories. Finally, it identifies future directions for research applying the theory in the physical activity domain.