Despite frequent statements to the contrary, there are good animal models for AIDS. In this review, we summarize the properties of one of the most useful animal models: infection of rhesus monkeys with simian immunodeficiency virus (SIV). The SIVs are an extensive group of HIV-related lentiviruses of nonhuman primates. They closely resemble the human AIDS viruses, HIV-1 and HIV-2, in both genetic sequence and biological properties. Some SIV isolates, most notably those derived from macaques and mangabeys, induce AIDS in rhesus monkeys in a time frame suitable for laboratory investigation. Rhesus monkeys are not endangered in the wild, they breed well in captivity, and they are available in reasonably large numbers. Study of SIV has already resulted in seminal contributions regarding the origins of the HIVs, AIDS pathogenesis, and vaccine and therapy research. Continued use of SIV systems will be an important weapon in our arsenal against AIDS.