Despite growing recognition of the problem, the obesity epidemic continues in the U.S., and obesity rates are increasing around the world. The latest estimates are that approximately 34% of adults and 15–20% of children and adolescents in the U.S. are obese. Obesity affects every segment of the U.S. population. Obesity increases the risk of many chronic diseases in children and adults. The epidemic of obesity arose gradually over time, apparently from a small, consistent degree of positive energy balance. Substantial public health efforts are being directed toward addressing obesity, but there is not yet clear evidence of success. Because of the complexity of obesity, it is likely to be one of the most difficult public health issues our society has faced.