AbstractThe following chapter reviews some aspects of amino acid and protein metabolism in animals. It attempts to provide the reader with up-to-date knowledge by discussing the first principles in the nutritional classification of amino acids. Amino acids is discussed in terms of comparative requirement of different species of animals, synthesis and catabolism, roles in intermediary nitrogen and energy metabolism, roles in health and disease, regulators of protein metabolism and protein efficiency and environmental pollution. A perspective on the challenges ahead and the lead role that amino acid nutrition and metabolism will be expected to take in this postgenomic era is presented.