1. An excess of subcutaneous fat in proportion to lean body mass. Excess fat accumulation is associated with increase in the size (hypertrophy) as well as the number (hyperplasia) of adipose tissue cells. Obesity is variously defined in terms of absolute weight, weight:height ratio, distribution of subcutaneous fat, and societal and esthetic norms. Measures of weight in proportion to height include relative weight (RW, body weight divided by median desirable weight for a person of the same height and medium frame according to actuarial tables), body mass index (BMI, kg/m2) and ponderal index (kg/m3). These do not differentiate between excess adiposity and increased lean body mass. In contrast, subscapular and triceps skinfold measurements and determination of the waist:hip ratio help define the regional deposition of fat and differentiate the more medically significant central obesity from peripheral obesity in adults. No single cause can explain all cases of obesity. Ultimately it results from an imbalance between energy intake and energy expenditure. Although faulty eating habits related to failure of normal satiety feedback mechanisms may be responsible for some cases, many obese people neither consume more calories nor eat different proportions of foodstuffs than nonobese persons. Contrary to popular belief, obesity is not caused by disorders of pituitary, thyroid, or adrenal gland metabolism. However, it is often associated with hyperinsulinism and relative insulin resistance. Studies of obese twins strongly suggest the presence of genetic influences on resting metabolic rate, feeding behavior, changes in energy expenditures in response to overfeeding, lipoprotein lipase activity, and basal rate of lipolysis. Environmental factors associated with obesity include socioeconomic status, race, region of residence, season, urban living, and being part of a smaller family. The prevalence of obesity is greater when weight is measured during winter rather than summer. Obesity is much more common in the southeastern U.S., although the northeastern and midwestern states also have high rates, a phenomenon independent of race, population density, and season.