Due to high demand to access biologically important oligosaccharide structures, efficient and stereoselective synthetic methods are required. This article addresses these two issues from our recent studies. The first part deals with β-mannosylation and α-sialylation known as the two most difficult glycosylation reactions which were now made possible by means of an “intramolecular aglycon delivery” system and control by “auxiliary”, respectively. In the second part, the remarkably efficient “orthogonal glycosylation strategy” which was developed based on tactical analysis of oligosaccharide synthesis is described.