Regular physical activity has many health benefits, but in spite of the benefits many Americans are not sufficiently active. There is increasing recognition of the importance of environmental factors–including the built environment–to help promote physical activity. The built environment describes physical or manmade features such as sidewalks, streetlights, traffic and parks that may promote or discourage activity. This synthesis examines the evidence on the built environment–proximity to destinations, sidewalks, aesthetics, access to parks and open spaces, and the walkability of the community–and the relationship to increased physical activity. Findings include: There is reasonably strong evidence of an association between many factors of the built environment and increased physical activity, but the evidence on whether or not the built environment promotes activity is weak. Most research shows that individual and societal factors are stronger drivers of activity than the built environment.