Obesity results from an imbalance between energy intake and energy expenditure. Human studies indicate that obese individuals have an increased basal metabolic rate secondary to an increased fat-free mass. A blunted dietary thermogenesis is observed but is not of sufficient magnitude to lead to a major weight gain. Indirect evidence suggests that physical activity may be low in obese individuals. In healthy lean subjects, overfeeding leads to a stimulation of spontaneous physical activity. The ensuing increase in energy expenditure may play a role in the prevention of weight gain. This response, however, shows a high interindividual variability. There is overall little evidence that major alterations of energy expenditure are present in obese individuals. It is likely that an alteration of mechanisms of food intake control plays a prominent role in the pathogenesis of this disease.