The increasing prevalence of childhood obesity, persistence of obesity over time, and health risks associated with it place childhood obesity among the highest national health concerns. The extent to which excess caloric intake or insufficient energy expenditure contribute to childhood obesity has not yet been clearly delineated. This report examines the components of the energy alance equation, focusing on the major components of energy expenditure (rest, thermic effect of food, and physical activity) whereby differences may affect energy balance and promote or perpetuate obesity among children. Some implications of differences in energy expenditure to childhood obesity and areas of needed research are suggested.