A common feature of many bacteria is the expression of a layer of extracellular polysaccharide usually organised into a discrete structure termed the capsule. The expression of a capsule results in the coating of the bacterium in a hydrated shell of high molecular weight polysaccharide molecules that mediate interactions between the bacterial cell and its immediate environment. The ubiquity of capsule expression across a diverse array of bacterial genera suggests that capsule expression is advantageous in a number of scenarios including infections of plants and animals and maintenance in a broad range of ecosystems. In this chapter we discuss the roles of polysaccharide capsules in the biology of a number of bacterial species and describe how expression of capsule gene expression is achieved in a broad range of bacterial pathogens. © 2010 Springer-Verlag Berlin Heidelberg.