Twenty years after it was first recognized, the HIV/AIDS epidemic continues to expand, but its impact varies greatly in different parts of the World. The worst of the epidemic is now centered in developing countries, especially sub-Saharan Africa, and areas such as Eastern Europe, which was only marginally involved a few years ago but has recently experienced the largest growth in the epidemic. In industrialized countries Highly Active Antiretroviral Therapy (HAART) has changed the natural history of HIV/AIDS, causing a reduction in mortality and morbidity due to HIV/AIDS and related diseases. Many interlocking factors determine the impact of HAART at the population level, including reduction of morbidity and mortality, changes in the natural history of HIV/AIDS and associated illnesses, and the effects of HAART on HIV transmission. To fully appreciate the potential benefits of HAART, the epidemic should continue to be monitored in the future, and the effects of HAART on reducing HIV transmission should also be evaluated. Interventions addressed to encourage the adoption of safer sex practices are badly needed, since a "rebound" in risky sexual behaviors was recently reported among high risk groups, which is, at least in part, attributable to the optimism about new treatments.