The essential amino acids are the amino acids that are essential for growth and health but cannot be synthesised by the body — they must come from food. The classic list counts eight for adults — isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine — with histidine essential for infants and arginine conditionally essential in stress and illness. A protein that supplies them all in adequate proportions is “complete” — animal proteins generally qualify, most plant proteins fall short in one, the limiting amino acid. The concept organises all of protein nutrition: complementing plant proteins (rice with beans, the classic pairing), setting protein requirements, and judging protein quality. They are called essential not because they are more important than the others, but because the diet is their only source.