Pneumococcal diseases are a major public-health problem all over the world. The etiological agent, Streptococcus pneumoniae (the pneumococcus) in surrounded by a polysaccharide capsule. Differences in the composition of this capsule permit the serological differentiation between about 90 capsular types, some of which are frequently associated with pneumococcal disease, others rarely. Invasive pneumococcal infections include pneumonia, meningitis, and febrile bacteremia; among the common non-invasive manifestations are otitis media, sinusitis, and bronchitis. At least one million children die of pneumococcal disease every year, most of these being young children in developing countries. In the developed world, elderly persons carry the major disease burden. Conditions associated with increased risk of serious pneumococcal disease include HIV infection, sickle-cell anaemia, and a variety of chronic organ failures. Vaccination is the only available tool to prevent pneumococcal disease. The recent development of widespread microbial resistance to essential antibiotics underlines the urgent need for more efficient pneumococcal vaccines. Immunity following pneumococcal disease is directed primarily against the capsular serotype involved. The currently licensed pneumococcal vaccine is based on the 23 most common serotypes, against which the vaccine has an overall protective efficacy of about 60% to 70%. Children aged < 2 years, and persons suffering from various states of immunodeficiency, for example HIV infection, do not consistently develop immunity following vaccination, thus reducing the protective value of the vaccine in some major target groups for pneumococcal disease. However, in the healthy elderly population, the polysaccharide vaccine provides relatively efficient protection against invasive pneumococcal disease. Extensive clinical trials are now under way with a new generation of pneumococcal vaccines. These protein-polysaccharide combinations, known as conjugate vaccines, contain 7-11 selected polysaccharides bound to a protein carrier, and induce a T-cell dependent immune response. These vaccines are likely to be protective even in children < 2 years of age, and may reduce pneumococcal transmission through a herd effect.