The prevalence of obesity has markedly increased in the past few decades, and this disorder is responsible for more health care expenditures than any other medical condition. The greater the body mass index (BMI) (calculated as weight in kilograms divided by the square of height in meters), the greater the risk of comorbidities, including diabetes mellitus, hypertension, obstructive sleep apnea, many cancers, dyslipidemia, cardiovascular disease, and overall mortality. Class III (extreme) obesity, defined as a BMI of 40 kg/m2 or greater, has also increased such that it now affects almost 1 in 20 Americans. The prevalence of extreme obesity is greater among women than among men and greater among blacks than among non-Hispanic whites or Hispanics. The effect of extreme obesity on mortality is greater among young than among older adults, greater among men than among women, and greater among whites than among blacks. The current permissive environment that promotes increased dietary energy intake and decreased energy expenditure through reduced daily physical activity coupled with genetic susceptibility is an important pathogenic factor. The number of bariatric surgical procedures performed annually is relatively small but increasing.