Evaluation and delivery of physical activity and exercise programs appear impeded by the substantial numbers of Americans who are unwilling or unable to participate regularly in physical activity. As a step toward identifying effective interventions, we reviewed available research on determinants relating to the adoption and maintenance of physical activity. We categorized determinants as personal, environmental, or characteristic of the exercise. We have considered supervised participation separately from spontaneous activity in the general population. A wide variety of determinants, populations, and settings have been studied within diverse research traditions and disciplines. This diversity and the varied interpretation of the data hinder our clearly summarizing the existing knowledge. Although we provide some directions for future study and program evaluation, there is a need for research that tests hypotheses derived from theoretical models and that has clear implications for intervention programs. We still need to explore whether general theories of health behavior or approaches relating to specific exercises or activities can be used to predict adoption and maintenance of physical activity.