Increased portion sizes of foods high in energy density (calories per gram of food) have been implicated in the obesity epidemic. Numerous studies show that children and adults eat more from larger portions of food than they do from smaller portions, a response known as the portion size effect. Despite the robust and consistent nature of these findings, the mechanisms underlying the portion size effect are not known. The long-term goal of this research is to identify the neural mechanisms involved in the portion size effect so that this information can be used to develop effective weight-management strategies. Differences in neural response to food cues, as demonstrated by functional magnetic resonance imaging (fMRI), can help clarify the factors that determine susceptibility to large portions. The goal of this study is to identify brain regions activated in response to portion size and energy density and relate these neural responses to laboratory eating behaviors in children.