Glycosylation is a common posttranslational modification of proteins in eukaryotes. The oligosaccharide components of glycoproteins can profoundly influence proteins' structure and in vivo biological activities, and are also involved in many important biological processes such as cell adhesion, pathogen infection, and immune response. It becomes clear that different sugar chains can distinctly influence a glycoprotein's property in terms of its serum's half life and in vivo activity. Therefore, getting the glycosylation right is important both for structure-function relationship studies and for biomedical applications of glycoproteins. Nevertheless, as glycosylation is not under direct genetic control, glycoproteins are typically produced as a mixture of glycoforms that differ in the oligosaccharide portions, from which homogeneous glycoforms are difficult to obtain. To address this problem, we have been exploring the endoglycosidase (ENGase)-catalyzed transglycosylation for glycopeptide and glycoprotein synthesis. ENGases hydrolyze N-glycans from glycoproteins, but some ENGases possess transglycosylation activity and are able to transfer a releasing oligosaccharide to a GlcNAc-containing peptide/protein acceptor in a single step, thus providing a convergent synthetic approach. But the transglycosylation yield is usually low (5-10%) due to the inherent hydrolytic activity. We have found that the use of synthetic oligosaccharide oxazolines, the presumed transition state mimics, as donor substrates not only expanded the substrate availability, but also resulted in substantial enhancement of transglycosylation, thus allowing the synthesis of both natural and tailor-made glycopeptides in excellent yield. The method has been successfully extended to glycosylation engineering of glycoproteins. The convergent chemoenzymatic approach allows totally independent manipulations of the oligosaccharide and protein portions, thus providing a solution to the long standing problem of �incompatibility� in protecting group manipulations in chemical glycoprotein synthesis. Applications of the chemoenzymatic method for synthesizing glycopeptide-based HIV vaccines will also be discussed.