The UDP-Glucuronosyltransferases (UGT) comprise a family of enzymes that detoxify and enhance the urinary excretion of a wide variety of xenobiotic and endogenous substrates by transferring glucuronic acid to sulfhydryl, hydroxyl, aromatic amino, or carboxylic acid groups. They have been subdivided into two families, UGT1 and UGT2, based on the evolutionary divergence of their genes. The enzymes of the UGT1A family play an important role in the metabolism of dietary constituents, phenols, and therapeutic drugs, and also the glucuronidation of bilirubin and iodothyronines. The enzymes of the UGT2B family are involved in the metabolism of bile acids, phenol derivatives, catecholestrogens and steroids. Although it is widely recognized that the liver is the major site of glucuronidation, it is now clear that UGT enzymes are also found in extra-hepatic tissues.