Interest in the lentivirus subfamily of retroviruses has greatly intensified due to the realization that HIV-1 and HIV-2 are members of this previously obscure group. Related lentiviruses have now been isolated from sheep, goats, horses, cattle, cats, monkeys, and humans. This issue of CTMI is devoted to the lentiviruses of nonhuman primates, referred to as simian immunodeficiency viruses (SIVs). The SIVs provide valuable tools for our quest to understand and control the HIVs, which are obviously important new human pathogens. Included in this volume are discussions of the distribution and molecular phylogeny of the SIVs and their use as animal models for the study of AIDS pathogenesis, and the chapters clearly illustrate how SIV models are contributing to our understanding of the ability of host immune responses to control infection at least temporarily and the ability of virus to evade these host immune defenses.