Although intravenous drug use is the major route of transmission in several countries, sexual transmission is the dominant mode of HIV spread globally, with a concomitant epidemic in infants borne to HIV-infected mothers. The HIV epidemic varies substantially from one geographic area to another, and three broad epidemic categories describe the diversity of features observed globally: low epidemic settings, centrzated epidemics, and generalized epidemics. The introduction of highly active antiretroviral therapy in industrialized countries has transformed AIDS from an inevitably fatal condition to a chronic, treatable condition, but this goal has yet to be realized in most resource-constrained settings that bear a disproportionate burden of infection. This article describes the modes of HIV transmission, geographic distribution of the evolving AIDS pandemic, and case studies of each of the three types of HIV epidemics, and presents global trends in AIDS and mortality.