In light of the well-documented health benefits of physical activity and the fact that the majority of adult men and women are inactive, promoting regular physical activity is a public health priority. This chapter reviews current research findings regarding the determinants of exercise behavior. It also discusses the implications of this knowledge for individual and public health recommendations and intervention strategies for promoting physical activity. The discussion is predicated on the belief that physical activity is a complex, dynamic process. During their lives, individuals typically move through various phases of exercise participation that are determined by diverse factors. This chapter discusses physical activity determinants in two broad categories: individual characteristics, including motivations, self-efficacy, exercise history, skills, and other health behaviors; and environmental characteristics such as access, cost, and time barriers and social and cultural supports.