The aim of this essay is to provide an overview of initiatives designed to increase physical activity among different populations and in different settings, and to set the context for the major challenges that lie ahead. The decline in habitual physical activity with modernization, and the causal link between physical activity and health are briefly reviewed. The need to understand physical activity as a health behavior and examples of behavior change theories that have been applied to the promotion of physical activity are discussed. Diverse projects and campaigns in three countries, i.e. South Africa, Australia and the US, are highlighted. Common themes found in these physical activity initiatives include the development of a theory-driven research base, inclusion of behavioral and social scientists on multidisciplinary teams, and advocacy for environmental changes that promote physical activity. Within the next decade, research projects and national campaigns such as those described herein will yield important new findings on how to increase physical activity levels among targeted sectors of the population. The research agenda for the future includes development of both basic and applied research on physical activity, and the integration of theory across social, behavioral and biomedical disciplines.