With the growing biological importance of oligosaccharides involved in glycoconjugates, practical synthetic methods for obtaining oligosaccharides have been required. Enzyme-catalyzed oligosaccharide synthesis has been shown to be one of the most effective methods for large-scale oligo-saccharide synthesis. This review describes enzyme-catalyzed synthesis of di, tri, and tetrasaccharide units, which are related to biological roles: (1) galactose-containing disaccharides, (2) fucosecontaining trisaccharides, (3) core oligosaccharides in mucin-type carbohydrate, and (4) human milk oligosaccharides. Such well-defined oligosaccharide units are useful as substrates for glycosidases and glycosyltransferases, and as starting materials for artificial glycopolymers, which are valuable as probes of carbohydrate-protein interaction.