We examine the impact of neighborhood socioeconomic conditions and "built environments" on obesity and overweight prevalence among U.S. children and adolescents using the 2007 National Survey of Children's Health. The odds of a child's being obese or overweight were 20-60 percent higher among children in neighborhoods with the most unfavorable social conditions such as unsafe surroundings; poor housing; and no access to sidewalks, parks, and recreation centers than among children not facing such conditions. The effects were much greater for females and younger children; for example, girls ages 10-11 were two to four times more likely than their counterparts from more favorable neighborhoods to be overweight or obese. Our findings can contribute to policy decisions aimed at reducing health inequalities and promoting obesity prevention efforts such as community-based physical activity and healthy diet initiatives.