Drug addiction is a major risk factor for HIV infection for an estimated 3.4-5.5 million injecting drug users (IDU) in more than 125 countries. In the USA, injecting drug use now accounts for nearly one-third of new AIDS cases [4] and for as many as three-quarters of annual incident HIV infections [5]. Behaviors associated with drug use that are responsible for HIV transmission include shared use of injection equipment and other drug paraphernalia, and unprotected vaginal and anal intercourse with an injecting drug user. Interventions that can reduce the prevalence of these behaviors are therefore a critical component of comprehensive AIDS prevention policy. Approaches to HIV risk reduction among IDU include drug abuse treatment, HIV testing and counseling programs, street-based outreach conducted by peer educators, individual and group counseling, community-level interventions to change IDU norms concerning safer injection and safer sex, and syringe exchange programs to provide IDU with sterile injection equipment. Syringe exchanges have been established widely in some developing, and a majority of the developed countries, [3] including the USA, although a Congressional funding ban still prevents them from receiving US government support. Although evidence indicates that syringe exchange is effective in slowing the spread of HIV in IDU socially conservative American politicians have embraced contrary evidence from two Canadian cities [9,10] to assert that syringe exchanges are not only immoral because they encourage drug use, but may also actively spread HIV [2]. The purpose of this review is to provide a comprehensive, critical review of published evidence of the effectiveness of syringe exchange programs in reducing HIV risk behavior and HIV seroconversion among IDU. While five previous reviews have been published, four are now seriously out of date, and a fifth [11] did not attempt a comprehensive critical review of the published literature. We have identified and will describe and discuss 42 studies published between 1989 and the end of 1999 that evaluated syringe exchange effectiveness.