Biotechnology associated with synthesis of glycopeptides and glycoproteins has recently advanced as glycotechnology. Studies toward glycotechonology include the artificial modification of sugar chains in glycoconjugates to improve their function because the physiological importance of sugar chains in living organisms is well recognized. Methods involving addition of oligosaccharides to peptides and proteins have attracted attention as efficient techniques in glycotechnology, especially those involving the transglycosylation activities of microbial endoglycosidases. The exploration of oligosaccharide oxazolines as donor substrates for the transglycosylation of endoglycosidases has significantly enhanced the efficiency of these processes. Moreover, discovery of novel endoglycosidase mutants with glycosynthase-like activity has made it possible to effectively synthesize large quantities of glycopeptides, as well as homogeneous glycoprotein. The use of mutant enzymes and oligosaccharide oxazolines has led to development of practical applications for the synthesis of bioactive glycopeptides and therapeutic glycoproteins as bio-medicines.