Although the disorders associated with obesity have been extensively studied, little attention has been paid to the fact that obesity is itself a chronic disease. This misunderstanding of the nature of obesity has contributed to the stigmatization of obese persons and to the use of inappropriate or inadequate treatment regimens. Although the etiology of obesity is still unclear, genetic, metabolic, and social factors are all believed to play a role in its development and progression. Behavioral therapy, exercise, very-low-calorie diets, drug therapy, and surgery affect the treatment of obesity of differing levels of severity. The regaining of weight following treatments other than surgery is very frequent, in part because periods of weight loss are rarely followed by maintenance programs. An increasing awareness of the chronic, multifactorial nature of obesity will ideally lead to the development of new long-term treatment programs that are safe and effective. Such programs are urgently needed in light of new data that show that the prevalence of obesity is increasing in the United States, as much as 30% in the last decade.