The energizing balance of the body is the result of intake and output of energy. Intake corresponds to eating and output corresponds to basal metabolism, physical exercise and heat production. Recent studies conducted on man suggest that obesity could be associated with a lack of heat production. Thus, a decreased thermogenic consumption associated with eating could contribute to the development of obesity. This hypothesis seems valid as far as obese rodents are concerned. In this case, it is a known fact that a specific organ, the brown fatty tissue, main effector of the thermogenesis induced by cold and eating. The molecular mechanisms of heat production by the brown fatty tissue, have been recently explained. Brown adipocytes are present in man, but their specific contribution to the energizing balance or imbalance is not yet established. This will be subject of further studies.