Glycosylation is a common and complex form of post-translational protein modification. Although a large and increasing number of unique structures is known to exist, most arise from a series of common synthetic intermediates and differ at their periphery. Glycosyltransferases, which recognize both the oligosaccharide acceptor and features of the underlying protein, may account for the synthesis of many unique oligosaccharides, particularly those associated with biologic functions dependent on specific oligosaccharide structures. UDP-Glc: glycoprotein glucosyltransferase, UDP-N-acetylglucosamine:lysosomal enzyme N-acetylglucosamine-1-phosphotransferase, and UDP-GalNAc:glycoprotein hormone N-acetylgalactosaminyltransferase are examples of glycosyltransferases that display peptide specificity. The features of peptide recognition are distinct for these three transferases and provide insights into the range of properties that can be expected for such transferases. Peptide-specific glycosyltransferases promise new insights into the regulation of glycosylation and its numerous biologic functions. They will also ultimately provide tools for engineering glycoproteins bearing specific oligosaccharide structures.