Glycosyltransferases catalyze glycosidic bond formation using sugar donors containing a nucleoside phosphate or a lipid phosphate leaving group. Two structural folds, GT-A and GT-B, have been identified for the nucleotide sugar-dependent enzymes, but other folds are now appearing for the soluble domains of lipid phosphosugar-dependent glycosyl transferases. Donor sugar substrates are most commonly activated in the form of nucleoside diphosphate sugars (e.g., UDP Gal, GDPMan); however, nucleoside monophosphate sugars (e.g., CMP NeuAc), lipid phosphates (e.g., dolichol phosphate oligosaccharides), and unsubstituted phosphate are also used. Nucleotide sugar-dependent glycosyltransferases are often referred to as Leloir enzymes. The acceptor substrates utilized by glycosyltransferases are most commonly other sugars but can also be a lipid, protein, nucleic acid, antibiotic, or another small molecule. Glycosyl transfer most frequently occurs to the nucleophilic oxygen of a hydroxyl substituent of the acceptor. However, it can also occur to nitrogen, sulfur, and carbon nucleophiles.