The development of a safe, effective preventive vaccine for human immunodeficiency virus (HIV) infection remains an area of vigorous research. Several highly innovative vaccine candidates are being developed, and more than 13 vaccine candidates have been tested in human phase I or II trials. All have produced antibody and several have produced modest neutralizing titers, but to date no reproducible evidence has suggested prolonged, high-titer neutralization across a diversity of HIV strains. Furthermore, only the live recombinant vector approaches have produced some evidence of cytotoxic T-cell responses. The principal obstacle to progress is the lack of definitive information on what constitutes a protective immune response. There is no animal model for HIV-induced disease. Hence, the identification of the correlates of immunity and more useful animal models is among the highest priorities for HIV vaccine research. Large-scale efficacy trials raise daunting scientific, ethical, and resource issues. Nonetheless, preparation in such trials is underway in order to be in a position to evaluate the most promising vaccine candidate.