Streptococcus pneumoniae is one of the major bacterial causes of invasive disease with significant morbidity and mortality. The ability of pneumococci to cause disease is directly related to the production of a capsule that promotes evasion of the host immune system of the patient. A large variety of capsular types can be produced by pneumococci but only a small number of these cause the majority of invasive disease. In February 2000, a protein-polysaccharide conjugate vaccine against the serotypes 4, 6B, 9V, 14, 18C, 19F, and 23F (PCV7) was licensed for use in infants and young children. This vaccine has to be safe, immunogenic, and effective in preventing pneumococcal disease in children. The pneumococcal conjugate vaccine also prevents asymptomatic colonization of the nasopharynx by PCV7 serotypes. It was expected that the broad usage of the vaccine would significantly reduce the incidence of invasive pneumococcal disease and the levels of pneumococcal resistance significantly because of the high prevalence of antimicrobial resistance of the PCV7 serotypes. Good results for this vaccine have led the World Health Organization to recommend the introduction of PCV7 into national immunization programs worldwide. However, important questions have recently been raised regarding the effectiveness of PCV7, especially if serotype replacement limits the efficacy of this vaccine. Recently, several authors have reported an increase in infections caused by non-PCV7 serotypes. In addition, resistance to commonly used antimicrobials has significantly increased among nonvaccine serotypes. Virulent pneumococcal serotypes not covered by the PCV7 vaccine and serotype replacement make it essential that there be continued monitoring and surveillance of pneumoccocal colonization and invasive disease.