It is proposed that body weight, like body water and body temperature, is physiologically regulated. In the case of body weight, coordinated adjustments in both the intake and expenditure of energy serve to stabilize the weights of individuals at a specified level and to resist their displacement from this level. Obese individuals also display these behavioral and metabolic adjustments to weight perturbations and thus appear to actively resist efforts to reduce their weight from the elevated levels they ordinarily display. Experimental studies of genetically transmitted and diet-induced forms of obesity in animals similarly suggest a view of obesity as a condition of body energy regulation at an elevated set-point. An individual's set-point for regulated body weight is apparently adjustable, shifting over a lifespan in conjunction with naturally occurring but still unspecified physiologic changes. Experimentally, the set-point for body weight can be adjusted by manipulation of specific hypothalamic sites. Lesions of the lateral hypothalamus, for example, cause a chronic reduction in the level at which laboratory animals regulate body weight. It thus appears that hypothalamic mechanisms play a primary role in setting the level at which individuals regulate body weight, and it is likely that the genetic, dietary and other lifespan influences on body weight are expressed through these mechanisms.