Approximately two thirds of U.S. adults and one fifth of U.S. children are obese or overweight. During 1980–2004, obesity prevalence among U.S. adults doubled, and recent data indicate an estimated 33% of U.S. adults are overweight (body mass index [BMI] 25.0–29.9), 34% are obese (BMI ≥30.0), including nearly 6% who are extremely obese (BMI ≥40.0). The prevalence of being overweight among children and adolescents increased substantially during 1999–2004, and approximately 17% of U.S. children and adolescents are overweight (defined as at or above the 95% percentile of the sex-specific BMI for age growth charts). Being either obese or overweight increases the risk for many chronic diseases (e.g., heart disease, type 2 diabetes, certain cancers, and stroke).