This chapter discusses tandem mass spectrometry in structural characterization of oligosaccharide residues in glycoconjugates. In tandem mass spectrometry (MS/MS), two-stage mass analysis is performed. In the first stage (MS-l), a specific ion of interest is selected according to its mass from the set of ions that constitute the conventional mass spectrum. The second stage (MS-2) generates, from this selected precursor ion, a spectrum of product ions that arise from metastable ion (MI) or collision-induced dissociation (CID). The discussion in this chapter is limited to the structural analysis of small carbohydrate subunits (up to 5 sugar units) attached to larger aglycon moieties. At present, MS/MS of sugar ions formed from glycoconjugates appears to fulfill some, but not all, of the basic requirements of structure analysis.