Dietary fat may play an important role in promoting overeating and the development of obesity because (1) dietary fat is converted to body fat at a low energy cost, (2) fatty foods are highly palatable, and (3) fat is less satiating, calorie for calorie, than carbohydrate and protein. There is some experimental evidence to support all of these suggestions. However, more studies are required, in particular to evaluate a recent hypothesis which proposes that the satiating power of dietary fat is least when it is eaten in the context of a high carbohydrate, energy-rich diet. Further findings indicate that the appetite control sysem is responsive to energy under-supply but is tolerant of oversupply (i.e. overeating), and this is perhaps another important factor contributing to the high incidence of obesity in economically developed countries.