Publisher Summary This chapter discusses oligosaccharides. The oligosaccharides comprise a large group of polymeric carbohydrates, each member consisting of relatively few monosaccharide units, which on complete acid hydrolysis yield only simple sugars or derivatives. On the-basis of the number of monosaccharide residues per mole, the oligosaccharides are classified as disaccharides, trisaccharides, tetrasaccharides, pentasaccharides, etc. No sharp distinction can be drawn between the oligosaccharides and the polysaccharides. The structures are similar and the oligosaccharides become polysaccharides as the number of combined sugar units increase. In a disaccharide, one monosaccharide residue is combined by an oxygen bridge between its hemiacetal hydroxyl and the hydroxyl of another residue. If hemiacetal hydroxyls of both monosaccharide residues are utilized for the glycosidic linkage, as in trehalose, the resulting disaccharide has no reducing group. The constituent monosaccharide units of an oligosaccharide may be different, as in lactose, which on hydrolysis gives one molecule each of D-glucose and D-galactose.