Vaccines are among the most powerful and cost-effective disease prevention tools available. A vaccine that could prevent HIV infection or stop progression of the disease would greatly help in the fight against the AIDS pandemic. Vaccines have been pivotal in worldwide smallpox elimination efforts, have nearly eliminated polio and have drastically reduced the incidence of infectious diseases like measles and pertussis in the US. A crucial question is whether a vaccine based on one strain of HIV would be effective for populations in which a different strain is predominant. There are also questions about how an HIV vaccine would protect individuals: the vaccine might not be able to actually prevent infection, but could prevent or delay progression to disease, or simply reduce the infectiousness of people who do become infected with HIV. HIV prevention education and counseling are important components of vaccine programs. Even after the release of a vaccine, there will be an ongoing need for effective behavioral prevention programs. An HIV vaccine will not be a “magic bullet” but it could play an extremely powerful role as part of a package of prevention interventions.