Overweight and obesity have increased dramatically in prevalence in the US. More than 66 percent of the US population is overweight (BMI ≥25) and 32 percent meet criteria for obesity (BMI ≥30). Obesity is particularly prevalent among some minority populations, such as African American, Hispanic, and Native American women. Overweight prevalence is also increasing in children, with more than 17 percent of children and adolescents considered overweight and an additional 16 percent at risk for overweight. Obesity is associated with numerous serious and chronic diseases, including type 2 diabetes, cardiovascular disease, sleep apnea, and certain forms of cancer. The causes of obesity are complex, and involve both genetic and environmental components. Environmental changes over the past two decades have increased sedentary behaviors, decreased physical activity, and increased consumption of more energy dense foods and larger portion sizes. Although an imbalance in energy consumption and expenditure are required to promote inappropriate weight gain, the relative contributions of each to the burgeoning obesity epidemic remain in dispute.