Obesity, defined as excess body fat, is detrimental to longevity. That obesity is a symptom rather than a disease is illustrated by its clinical and experimental heterogeneity. Triglyceride accumulation proceeds by enlargement of adipose cells or hyperplasia of the adipose organ, or both. Weight loss occurs by contraction of individual adipose cells without change in their total number. Food intake is controlled by the ventromedial and lateral hypothalamic areas, in response to not fully understood environmental and metabolic cues. Increased secretion of insulin and decreased secretion of growth hormone characterize obesity, and cortisol secretion and turnover are increased; these seem to be secondary to the obese state. The failure of dietary restriction has led to radical approaches for treating the grossly obese. Rapid and steady weight loss can be achieved by semistarvation or total fasting, but these methods are complicated by hypotension and electrolyte imbalances, as well as by metabolic derangements owing to protein loss and the use of body fat as the primary fuel source. Thyroid hormone can be useful in weight loss maintenance. Surgical approaches are a last resort; the commonest, the jejunoileal bypass, has produced substantial, lasting weight loss in many cases, but there is significant operative mortality and sometimes progressive liver failure and death, or intestinal obstruction and transitory arthropathy.