Inequalities in child obesity within and among nations result from unequal distribution of resources and environments that prevent unhealthy weight gain--healthy food, opportunities for physical activity, primary and preventive health care, and protection from stressors. While some developed nations have recently slowed the increase in child obesity, none has successfully reversed the growing concentration of child obesity among the poor and disadvantaged. This commentary reviews the evidence on patterns and causes of unequal distribution of child obesity in developed nations and analyzes the implications for the development of interventions to reduce these inequalities.