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...