The energy density (kcal/g) and the portion size (g) of foods have been identified as two properties of foods that can modulate energy intake. Recent studies have shown that when either the energy density or the portion size of foods is increased, energy intake increases. Within a meal, when both factors are increased simultaneously, their effects are independent and add together to increase energy intake. On the other hand, reducing the energy density of a first course, while increasing the volume that is consumed, leads to a decrease in energy intake at the entire meal. The mechanisms by which both factors exert their influence are not well understood but likely include cognitive and orosensory factors as well as physiological controls related to gastric distention and gastric emptying. Findings from studies in this area of research provide evidence that the energy density and the portion size of foods are important determinants of energy intake. An environment in which the food supply is ample and large portions of energy-dense foods are readily available to consumers can contribute to an overconsumption of calories.