Nitrogen balances have been conducted in preterm infants, preschool children, and 6- to 10-year-old children to determine dietary indispensable amino acid. A recent review concluded that the data, being sufficiently uncertain, could not be used as the basis for defining amino acid requirements in infants and children. Therefore, it was decided to use a factorial approach (basal plus growth). This approach is based on the assumption that basal requirements are the same throughout the life cycle. Recently, using indicator oxidation, the requirements of the eight classical indispensable amino acids have been defined in adults. These values have been used as the basal component of requirement in childhood. The growth component was based on the changes in body protein with age. We have recently shown that the maintenance requirements for branched chain amino acids are similar in children and adults, thereby validating the factorial approach.