The mechanisms that underlie successful initiation and adherence to physical activity regimens are not well understood. Few theoretical models have used consistent explanatory variables that are theory-driven and many findings that use extant models are equivocal. Social Cognitive Theory (SCT) as presented by Bandura (1986, 1997) appears to have strong promise as a guide to understanding physical activity behaviors and developing clinically relevant interventions to promote the initiation and maintenance of physical activity. This critical systematic review of research using SCT was completed to determine the predictive ability of model constructs in explaining physical activity behavior and in identifying key intervention components found to enhance physical activity initiation and maintenance. Following review for quality and adequacy, published research during the years 1990-1998 contained 27 studies that examined the relationship between the construct of SCT, self-efficacy, and physical activity. All of the descriptive studies found a statistically significant relationship between self-efficacy and exercise behavior. Intervention studies demonstrated that participation in an exercise program promoted self-efficacy, and that programs designed to increase outcome expectations and self-efficacy significantly increased exercise behavior. Due to the centrality of self-efficacy in many of the social psychological theories that help explain the attitude-intention-behavior triad, a strong need remains to design interventions to maximize its usefulness. Clear, generalizable, systematic and theoretically comprehensive, randomized, controlled studies are needed to understand the usefulness of the construct.