Essential amino acids are a group of nine amino acids that humans cannot synthesize and must obtain from diet: L-histidine, L-isoleucine, L-leucine, L-lysine, L-methionine, L-phenylalanine, L-threonine, L-tryptophan, and L-valine. They serve as building blocks for proteins, neurotransmitters, and metabolic intermediates. Each has distinct side chains conferring unique biochemical roles, such as branched-chain amino acids (leucine, isoleucine, valine) in muscle protein synthesis. They are highly soluble in water due to zwitterionic nature at physiological pH. In food science, they contribute to flavor (e.g., umami from glutamate precursors) and nutrition. Deficiency leads to impaired growth and metabolic disorders. They are generally recognized as safe (GRAS) when consumed in dietary amounts.