The polysaccharide capsule which surrounds bacterial species like Haemophilus influenzae, Streptococcus pneumoniae, Neisseria meningitidis, Salmonella typhi, is a potent virulence factor. It protects the bacterium from phagocytosis, but capsule specific antibodies plus complement binding to the capsule opsonise the organism for phagocytosis and elimination. Purified capsules elicit T-independent antibody responses without a memory function, and are often poorly immunogenic in infants where much of invasive H. influenzae type b (Hib) and pneumococcal infection is seen. Covalent linkage of the polysaccharide, or fractions thereof, to immunogenic carrier proteins creates glycoconjugates which are T-dependent antigens and which prime for boosting either with the glycoconjugate or the capsular polysaccharide; During the 1990s, four Hib glycoconjugate vaccines have been introduced and in countries that have vaccinated the majority of children, the success has been stunning. In countries with very high immunization coverage the disease has been virtually eliminated and, to a decline of over 95% in countries with slightly lower vaccine rates. Worldwide use of Hib glycoconjugate vaccines offers the possibility of elimination of invasive Hib disease. Pneumococcal (11 serotypes with coverage of approximately 85% of invasive disease) and meningococcal (A, C, W 135, Y but not B) glycoconjugates are in pre-registration phases and offer the prospect of being as successful as the Hib glycoconjugates.