The uridine diphosphate (UDP)-glycosyltransferases (EC2.4.21.17) are a group of enzymes that catalyze the transfer of sugars (glucuronic acid, glucose, and xylose) to a variety of acceptor molecules (aglycones). The sugars may be attached at aromatic and aliphatic alcohols, carboxylic acids, thiols, primary, secondary, tertiary, and aromatic amino groups, and acidic carbon atoms. In vivo, the most common reaction occurs by transfer of glucuronic acid moiety from UDP glucuronic acid (UDPGA) to an acceptor molecule. This process is termed either glucuronidation or glucuronosylation. When the enzymes catalyze this reaction, they are also referred to as UDP-glucuronosyltransferases (UGTs). The structure and function of the enzymes have been the subject of several reviews (1–4). This chapter reviews the role of these enzymes in drug-drug interactions that occur in humans.