The HIV / AIDS pandemic is a global emergency and a preventive HIV vaccine is urgently needed. HIV has, however, proved a difficult pathogen to vaccinate against. This is largely because HIV has a very high mutation rate and can escape immune responses, it has a latent stage where it can rest silently integrated into host DNA, and neutralising antibodies that can neutralise diverse field strains have so far proved difficult to induce. There is however, considerable evidence now that HIV-specific CD4 and CD8 T cells can provide partial control of HIV replication and delay or prevent disease. Technologies to quantify and analyse HIV-specific T cells have advanced recently, and in particular ELISpot, intracellular cytokine staining and tetramer studies have provided clear analyses of the ability of HIV vaccines to induce T cell responses. The use of pools of overlapping HIV peptides as in vitro antigens has also provided a standardised reagent for accurate measurement of T cell responses. HIV protein vaccines have not induced broad neutralising antibodies or T cell responses and failed to protect humans in the only phase III efficacy trial yet completed. Viral vectors, such as canarypox, engineered to express HIV genes, have induced HIV-specific CD8 T cell responses in a minority of subjects in phase II trials and are proceeding to human efficacy trials. Currently, the most effective method of inducing CD8+ CTL immunity in non-human primates utilises priming with naked plasmid DNA and then boosting with recombinant viral vectors both encoding various parts of the HIV genome. Such vaccines have induced non-sterilising immunity to virulent Simian / Human immunodeficiency virus exposure in macaques and have entered phase I trials. Multiple other approaches are also being evaluated in what has become a global effort for a vaccine to prevent AIDS. Although an HIV vaccine is still a long way off, there is reason to be optimistic that a vaccine to prevent AIDS will eventually be developed.