The indicator amino acid oxidation (IAAO) method is based on the concept that when 1 indispensable amino acid (IDAA) is deficient for protein synthesis, then all other IDAA, including the indicator amino acid, will be oxidized. With increasing intakes of the limiting amino acid, IAAO will decrease, reflecting increasing incorporation into protein. Once the requirement for the limiting amino acid is met, there will be no further change in the indicator oxidation. Originally, the IAAO method was designed to determine amino acid requirements in growing pigs. The minimally invasive IAAO method developed in humans has been systematically applied to determine IDAA requirements in adults. Due to its noninvasive nature, the IAAO method has also been used to determine requirements for amino acids in neonates and children, and in disease. The IAAO model has recently been applied to determine the metabolic availability (MA) of amino acids from dietary proteins and to determine total protein requirements. The IAAO method is robust, rapid, and reliable; it has been used to determine amino acid requirements in different species, across the life cycle, and in diseased populations. The recent application of IAAO to determine MA of amino acids and protein requirements is also very novel.