Primate lentiviruses infect target cells by interacting with the cell surface protein, CD4 and additional molecules, termed coreceptors. Recently, HIV-1 coreceptors have been identified as seven transmembrane spanning, G-protein coupled receptors of the chemokine receptor family. Thus, expression of CD4 and an appropriate coreceptor is both necessary and sufficient to render target cell permissive for fusion with virions or infected cells. The spectrum of tissue tropisms exhibited by primate lentiviruses can be largely explained by differential utilization and distribution of coreceptors. This article reviews what is currently known about the selective utilization of particular coreceptors by primate lentiviruses and the nature of the envelope/coreceptor interaction, with particular reference to two important HIV-1 coreceptors, CCR-5 and CXCR-4. It has become clear that these interactions are somewhat 'plastic': Variability is evident, both in the selection of coreceptor and the way in which different viral strains interact with their cognate coreceptors. The implications of these findings both for attempts to block HIV infection with coreceptor targeted agents and for understanding HIV replication in vivo is discussed.