OBJECTIVES: This study examined (1) descriptive patterns in perceived environmental and policy determinants of physical activity and (2) associations between these factors and behavior. METHODS: A cross-sectional study was conducted from 1999 to 2000 among US adults; individuals at lower income levels were oversampled. RESULTS: Availability of areas for physical activity was generally higher among men than among women. The 4 most commonly reported personal barriers were lack of time, feeling too tired, obtaining enough exercise at one's job, and no motivation to exercise. Neighborhood characteristics, including the presence of sidewalks, enjoyable scenery, heavy traffic, and hills, were positively associated with physical activity. There was a high level of support for health policy-related measures. Up to one third of individuals who had used environmental supports reported an increase in physical activity. CONCLUSIONS: An array of environmental and policy determinants, particularly those related to the physical environment, are associated with physical activity and should be taken into account in the design of interventions.