Assay methods for representative glycosyltransferases were described, covering those involved in the synthesis of glycosphingolipids, N-glycans, O-glycans, and proteoglycan glycosaminoglycans. In addition, intracellular localization of glycosyltransferase was comprehensively summarized. Lastly, complex formation of glycosyltransferase proteins with related molecules including subunits, chaperones, and enzyme regulators, that have been recently reported was also summarized.