Development of a preventive vaccine for HIV is the best hope of controlling the AIDS pandemic. HIV has, however, proved a difficult pathogen to vaccinate against because of its very high mutation rate and capability to escape immune responses. Neutralizing antibodies that can neutralize diverse field strains have so far proved difficult to induce. Adjuvanting these vaccines with cytokine plasmids and a "prime-boost," approach is being evaluated in an effort to induce both CTL and antibody responses and thereby have immune responses active against both infected cells and free viral particles, thereby necessitating fewer doses of recombinant protein to reach maximum antibodies titers. Although obstacles exist in evaluation of candidate HIV vaccines, evidence from natural history studies, new molecular tools in virology and immunology, new adjuvants, new gene expression systems, new antigen delivery systems, recent discoveries in HIV entry and pathogenesis, and promising studies of candidate vaccines in animal models have provided reasons to hope that developing a safe and effective AIDS vaccine is possible and within reach.