Theanine (γ-glutamyl-L-ethylamide) is the most abundant non-protein amino acid in tea leaves. In addition to Camellia sinensis, theanine occurs in several plants belonging to the Ericales. Biosynthesis of theanine from glutamic acid and ethylamine by theanine synthetase is present in all organs of tea seedlings, but roots are the major site of theanine biosynthesis in adult tea trees. Theanine is transported from roots to young leaves via the xylem sap. Theanine is hydrolysed to glutamic acid and ethylamine in leaves. Ethylamine produced from theanine is predominantly used for catechin biosynthesis. Concentration of ammonia and light intensity influence the biosynthesis and degradation of theanine, respectively. Biosynthesis, translocation and degradation of theanine and related enzymes and genes are reviewed.