The optimal fat intake for children is discussed in light of their energy requirements. Total energy requirements were estimated from doubly labeled water studies of total energy expenditure (TEE) and the energy cost of growth. Basal metabolic rates (BMRs) were calculated from weight by using the equations of Schofield et al or by indirect calorimetry. Activity energy expenditure and physical activity levels were calculated as TEE - BMR and TEE/BMR, respectively. Weight-specific energy requirements for maintenance and growth changed inversely to the increased energy needed for physical activity in healthy, active children. The total energy requirements of infants increased from 1.4 MJ/d at 1 mo to 4.0 MJ/d at 24 mo. The energy cost of growth decreased sharply from 37-38% to 2% of the total requirement during the first 24 mo of life. Energy requirements increased from 4 MJ/d at 2 y to 11 MJ/d at 18 y in girls and from 5 to 15 MJ/d in boys. The energy cost of growth varied between 1% and 4% of total energy requirements in childhood and adolescence. The current recommendation of 30% of energy from dietary fat for children aged >2 y is sufficient for adequate growth. Lower fat intakes may be associated with inadequate vitamin and mineral intakes and increased risk of poor growth. Diets higher in fat may lead to higher energy intakes and higher body fat, although available data for children are conflicting. Beyond infancy, children can meet their energy needs for maintenance, physical activity, and growth from a diet providing 30% of energy from fat.