New physiological discoveries are providing the behavioral sciences with a unique opportunity for the reconstruction of traditional theories of motivation. We are no longer constrained, for example, to view overeating solely as a result of an increased hunger drive. This apparent contradiction results from the demonstration that the hypothalamic centers mediating hunger and satiety have separate anatomical localizations and separable behavioral consequences. Overeating can thus theoretically result either from an increase in hunger drive or from an decrease in satiability. The first possibility has generally been accepted with little question. The second has now been demonstrated by experimental damage to the satiety centers of animals who thereupon present the paradox of an animal which eats itself into obesity through a hunger drive which is actually reduced in intensity. Recent clinical studies suggest that either decreased satiability or increased drive may occur in human obesity. Persons manifesting the "night-eating syndrome" report an inability to stop eating rather than any increased desire to eat. Obese persons who overeat in binges, on the other hand, report compelling urges to overeat at such times.