The treatment of obesity has until recently concentrated upon decreasing energy intake. Experiments using animal models of obesity have demonstrated that a defective thermogenic response to food ingestion is involved in the development of obesity. In man the evidence is less conclusive since there are as many studies which report a decreased thermogenesis in the obese as those which report a similar thermogenic response in obese and lean individuals. Evidence is given supporting the involvement of the sympathetic nervous system in the thermic response to glucose/insulin infusions in both lean and obese individuals and attempts to resolve the controversy as to why some studies report that the obese have a decreased and other studies a similar thermic response to nutrient administration.