The purpose of the NIAID/FDA joint workshop, "correlates of immunity for pneumococcal conjugate vaccines (PCVs)," was to discuss the present understanding of protective immunity against invasive pneumococcal disease and identify in vitro measures that may represent immunologic correlates in future clinical trials. Animal and clinical data support functional antibody as the basis for protection, but IgG antibody concentration has conventionally been the principle immunologic parameter for non-inferiority comparisons. No consensus for a pre-defined threshold antibody level was reached. Affinity maturation may contribute to protection, but its role has not been established. Opsonophagocytic activity, avidity and immunologic memory are important secondary measures to characterise functional antibody and long-term protective responses. Immunologic memory may also be useful for evaluation of new vaccine serotypes. More definitive qualitative and quantitative immunogenicity criteria for use by National Control Authorities still need to be established.