The energy density of foods and beverages is defined as the available energy per unit weight (kJ/g). Energy density of the diet is usually calculated excluding non-caloric beverages and drinking water. Because water contributes more to the weight of foods than any macronutrient, energy-dense foods are not necessarily those high in sugar or fat, but those that are dry. Evidence linking dietary energy density with body weight is critically evaluated in this review. Existing reports of a positive association between dietary energy density, higher energy intakes, and weight gain are based on laboratory and clinical studies. Although some cross-sectional epidemiological studies have linked dietary energy density with higher body mass index (BMI) values, the data are not consistent. At this time, there are no longitudinal cohort data linking dietary energy density with higher obesity risk.