This review describes data sources for assessing the prevalence of overweight and obesity in the entire U.S. population, and some examples of surveillance in specific subpopulations (i.e., schoolchildren, military, and athletes). On the basis of body mass index (ratio of weight in kg to height in m2) of 30 or higher from body measurement, which is more accurate than self-reported data, the prevalence of obesity among U.S. adults increased markedly in recent decades. Obesity prevalence has differed little by race-ethnicity among males but is higher in minority vs. white females. For waist circumference, a measure of abdominal obesity associated with various chronic diseases, the temporal increases in prevalence among U.S. adults are also striking. Obesity prevalence is higher in certain states and metropolitan areas in the South. Population surveillance of risk factors (e.g., diet, physical activity, and neighborhood characteristics) may be useful in interpreting temporal and geographic variation in obesity and obesity-related medical conditions.