Phenylalanine is an essential amino acid. It has a sparing action on Tyrosine. Tetrahydrobiopterin is an important cofactor for phenylalanine hydroxylase, deficiency of which can lead to inborn metabolic defect. Biologically important products from tyrosine are melanin, catecholamine’s and thyroid hormones. Tyrosinase requires copper, while tyrosine hydroxylase requires tetrahydrobiopterin as cofactor. L-Dopa, an inhibitor of DOPA decarboxylase is used in the treatment of Parkinson’s disease. The end product of epinephrine metabolism is Vanillylmandelic acid (VMA). Its excretion is increased in pheochromocytoma and neuroblastoma. Phenylketonuria (PKU) is an inborn error of metabolism, caused by defective Phenylalanine hydroxylase enzyme. The disease manifests with profound mental retardation, convulsions and hypopigmented skin. Alkaptonuria is due to deficiency of homogentisate oxidase. There is excretion of urine that darkens on standing. Absence of tyrosinase leads to ‘albinism’. Tryptophan produces biologically important products viz. Nicotinic acid, Serotonin and Melatonin. Serotonin is an important neurotransmitter. It is catabolized to 5 HIAA; it is increased in carcinoid tumors. Melatonin produced in the pineal gland, regulates the biological rhythms and sleep-wake cycle. Decarboxylation of histidine produces histamine. It is a stimulator of gastric secretion, both acid and enzymes. FIGLU excretion test is used to assess folic acid deficiency. Glucogenic amino acids are: Glycine, Alanine, Valine, Serine, Threonine, Aspartic acid, Glutamic acid, Asparagine, Glutamine, Methionine, Cysteine, Arginine, Histidine, and Proline. Ketogenic amino acids are; Leucine, Lysine. Partially glucogenic and partially ketogenic amino acids are: Phenyl alanine, Tyrosine, Tryptophan, and Isoleucine. Aminoaciduria’s are inborn metabolic disorders, which lead to mental retardation in most cases. Early detection and prompt management helps prevent complications.