Obesity has reached epidemic proportions in the United States, with 35.1% of adults being classified as obese. Obesity affects every segment of the US population and continues to increase steadily, especially in children. Obesity increases the risk for many other chronic diseases, including diabetes mellitus, cardiovascular disease, and nonalcoholic fatty liver disease, and decreases overall quality of life. The current US generation may have a shorter life expectancy than their parents if the obesity epidemic is not controlled, and there is no indication that the prevalence of obesity is decreasing. Because of the complexity of obesity, it is likely to be one of the most difficult public health issues our society has faced. © 2009 Elsevier Inc. All rights reserved.