Publisher Summary This chapter discusses bacterial polysaccharides. The bacterial polysaccharides make up a group of polymers in which the structural variation is almost unlimited, and unusual sugars are often the components of these polymers. Some bacterial polysaccharides are commercially important and produced industrially. Most of the bacterial polysaccharides are antigenic and some are used as vaccines. A majority of the bacterial polysaccharides show high immunological specificity and are produced by only one type, one species, or one group of bacteria. These polysaccharides are generally composed of oligosaccharide repeating units. Other bacterial polysaccharides do not show this specificity but may be elaborated by bacteria belonging to different groups or families. In addition to the polysaccharides proper, bacteria also produce other polymers in which carbohydrates are main components. Also a considerable number of monosaccharides have been identified as the components of bacterial polysaccharides and the number is increasing. Bacterial classification is complicated and is frequently changed. Lastly, even if structures have been assigned to only a small number of the bacterial polysaccharides, it nevertheless seems possible to make some generalizations concerning the frequency of different structural elements.