The present article focuses on in vitro synthesis of cellulose and related polysaccharides, in which polymer synthesis using an isolated enzyme as catalyst (`enzymatic polymerization') as the major part and conventional chemical synthesis are described. Typical natural polysaccharides such as cellulose, xylan and chitin having a b(1 ! 4) structure have been successfully synthesized for the ยฎrst time by using enzymatic polymerization. Hydrolases were most often employed as catalyst, which catalyze the bond-cleavage (hydrolysis) of polysaccharides in vivo, but the glycosidic bond-formation in vitro to give polysaccharides. All enzymatic polymerizations gave a polysaccharide having structure with perfect control of regio-and stereo-selectivity. A key issue for the polymerization to occur is the appropriate design of monomers which is considered to be close in structure to that of the hydrolysis transition state. This principle was extended to the synthesis of unnatural polysaccharides. In relation to the polymerization mechanism, fundamental mechanistic aspects of in vivo reaction of these enzymes are also discussed. Oligo and polysaccharide synthesis is achieved by using other classes of enzymes as catalysts, such as glycosyltransferases, phosphorylases and artiยฎcial glycosynthases prepared by mutation of glycosylases. Conventional chemical syntheses are based on polycondensation, ring-opening polymerization, and stepwise elongation. Some characteristics of these syntheses are described.