The widespread prevalence of physical inactivity in both adults and children presents a problem of great social significance in this country and around the world. The behavioral assessment and promotion of physical activity is an emerging area of research in which behavior analysis is well suited to undertake. Continued research in the assessment and promotion of physical activity is necessary for developing useful strategies to combat an increasingly sedentary population. The four papers in this symposium address the problem of physical activity in the following ways: 1) using contingency management to increase physical activity with adults, 2) comparing function-based interventions for children to interventions based on CDC and WHO recommendations, 3) assessing the effect of activity context on physical activity and activity preference with children with developmental disabilities, and 4) evaluating the ways that physical activity data are best analyzed and depicted.