For the first time in its five versions, Nutrition and Your Health: Dietary Guidelines for Americans contains an apparently nondietary guideline recommending physical activity. Although new as a separate guideline, physical activity has been included in the weight guideline of previous versions. The current version recognizes the importance of physical activity to health beyond its effect on weight maintenance. The purpose of this paper is to examine what data are available or required to evaluate the level of physical activity in the population, particularly in light of current recommendations. The physical activity sections of several national surveys that assess individual behavior or activity-related policies are described. Surveillance of physical activity as a risk factor for chronic disease is critical because physical inactivity is highly prevalent, strongly associated with increased morbidity and mortality, costly and preventable. Determinants of physical activity behavior are also considered. These determinants are potentially important factors for surveillance and are critical components for planning successful interventions.