The O-polysaccharides of bacterial lipopolysaccharides are in general regular, periodic polymers with diverse structures that contain, in many instances, comparatively rare monosaccharides. The oligosaccharides that constitute the repeating units of these polysaccharide antigens provide a demanding challenge in terms of glycoside synthesis, an objective which is particularly important since these structures act as antigenic determinants which are valuable markers of bacterial infection. Advances in glycoside synthesis together with the ancillary techniques of chromatographic separation and high resolution NMR spectroscopy have permitted rational synthesis of such oligosaccharides to be planned, successfully completed and have in a limited number of instances, even allowed small polysaccharides, representative of the O-polysaccharide, to be synthesized via polymerization of synthetic repeating units. The most intensive synthetic efforts have focused on the O-antigens of Salmonella, Shigella, and E. coli, although increasing attention is being given to the synthesis of antigenic determinants of other Gram-negative pathogens, including those of Brucella.