Schools can play an important part in the national effort to prevent childhood obesity. More than 95 percent of American youth aged 5 to 17 are enrolled in school, and no other institution has as much continuous and intensive contact with children during the first two decades of life. Schools can promote good nutrition, physical activity, and healthy body weight among children through healthful school meals and foods, physical education programs and recess, classroom health education, and school health services. In this article, school food and physical activity environments are analyzed, and federal, state, and local policies related to food and physical activity standards in schools are examined. Recommendations for school-based efforts to advance obesity prevention are also discussed.