The AIDS pandemic is a global emergency and a preventive vaccine is urgently needed. CD4 and CD8 T-cell responses appear important in controlling human immunodeficiency virus (HIV)-1 in humans and simian immunodeficiency virus (SIV) in macaques. The utility of vaccines that induce high levels of SIV- or HIV-specific T cells has recently become clearer. Since T cells recognize virus-infected cells rather than free virus, T-cell-based vaccines only have the capacity to control infections (non-sterilizing immunity) and to prevent continuing or persisting infection. An HIV/SIV infection of macaques that is partially controlled by vaccine-induced T-cell responses permits a critical window of opportunity for the efficient generation and recruitment of additional T- and B-cell immune responses to the incoming viral inoculum. Although CD8-depletion experiments in macaques have defined the utility of CD8 T responses in control of SIV infections in macaques, direct evidence on the utility of either CD4 or CD8 T-cell responses in protective immunity to SIV following vaccination is lacking. The availability of genetically identical macaques would allow cell transfer studies and help define with more certainty the role of cellular immune responses in protection from AIDS. The review also focuses on the development of syngeneic macaques by twinning and cloning technologies.