After the definition and the heredity of inborn errors of amino acid metabolism and a discussion of the incidence of these diseases, the four primary types of clinical features, namely 1) the prenatal, 2) neonatal (with acute onset), 3) mild, and 4) abortive types are described. The pathophysiology of brain damage is discussed. Based on recent findings, inborn errors of amino acid metabolism were classified according to their cause, 1) primary defect in catabolism, and 2) disturbances in the transport of amino acid. For each disease, the amino acids which can be detected in the plasma or in the urine are listed, along with the enzymes which are defective in each case.