Archaea, which form a separate realm of life than bacteria, often (but not always) live in extreme environment. Their amino acid supply is provided by import from the extracellular medium and by endogenous synthesis. Although studies on amino acid metabolism in archaea have been performed on a relatively limited number of species, it clearly appears that metabolic pathways involved in amino acid synthesis can differ between the different archaeal species. This indicates several variations among archaea for synthesizing the required 22 proteinogenic amino acids. Amino acids are used by archaea for the synthesis of macromolecules such as proteins, RNAs, and DNA. Some amino acids like glutamate and arginine are used for ATP supply in archaea, while amino acid-derived metabolites such as agmatine and spermidine are necessary for optimal growth. Other amino acid-derived metabolites including cadaverine, putrescine, nitric oxide, S-adenosylmethionine, and glycine betaine contribute to the capacity of archaea to face adverse physicochemical adverse conditions such as high temperature, high ammonia concentration, high osmolarity, and radiation-induced stress.