Obesity is considered to be a major nutritional disorder in the U.S. and in many parts of the industrialized world. The physiology of the obese and their propensity for chronic disease has been of growing interest over the past few years, and an extensive literature has begun to accumulate. Obesity is a heterogeneous disorder. When viewed in the broadest sense, it has been considered a disorder of energy balance. The development of obesity in humans is of complex etiology, involving genetic and environmental components that affect regulatory and metabolic events. The prevalence of overweight and obesity in a population depends on the particular reference or standard of desirable weight selected for use. A trend toward increasing height and weight has been evident among adults for several centuries, and among children as early as the 7th year of life in developed countries. Overweight persons are at increased risk for coronary artery disease, high blood pressure, diabetes mellitus, and cancer. The degree of overweight that carries additional risk without affecting mortality needs to be defined. Overweight most likely contributes in varying degrees to morbidity in different societies, because the risk for most common chronic diseases is multifactorial. In defining overweight and obesity, morbidity, in addition to mortality, ought to be taken into consideration. The multidisciplinary approach to the study of obesity--borrowing concepts and techniques from endocrinology, neurobiology, genetics, and nutrition--should yield new insights into how environmental factors such as diet and physical expenditure interact to influence energy metabolism and body composition.