Amino acids (AA) have enormous physiological importance, serving as building blocks for proteins and substrates for synthesis of low-molecular-weight substances. Based on growth or nitrogen balance, AA were traditionally classified as nutritionally essential or nonessential for animals. Although those AA that are not synthesized in eukaryotes (nutritionally essential AA, EAA) must be present in animal diets, nutritionally nonessential AA (NEAA) have long been ignored for all species. Emerging evidence shows that nonruminants cannot adequately synthesize NEAA or conditionally essential AA (CEAA) to realize their growth or anti-infection potential. Likewise, all preformed AA are needed for high-producing cows and rapidly growing ruminants. Many NEAA and CEAA (e.g., arginine, glutamine, glutamate, glycine, and proline) and certain EAA (e.g., leucine and tryptophan) participate in cell signaling, gene expression, and metabolic regulation. Thus, functions of AA beyond protein synthesis must be considered in dietary formulations to improve efficiency of nutrient use, growth, development, reproduction, lactation, and well-being in animals.