The chemokine receptors CCR5 and CXCR4 were identified as HIV-1 co-receptors in 1996. Since then, a range of agents that bind these receptors and potently block HIV-1 infection have been described, including monoclonal antibodies, peptides and modified chemokines. However, small organic molecules that bind CCR5 are currently the most promising of the co-receptor antagonists for the potential treatment of HIV. These agents are now in advanced stages of clinical development and should soon augment current therapies, as well as being candidates for inclusion in microbicides. Unlike existing drugs that target HIV proteins (eg, reverse transcriptase and protease), co-receptor antagonists bind receptors encoded by the host. As a consequence, blockade of these receptors may result in immunosuppressive effects or other disorders. Furthermore, co-receptor inhibitors may also be more toxic than currently available HIV therapies, and it is not yet clear whether they will become candidates for first-line therapy. Nonetheless, safer, less toxic versions of such inhibitors may be achievable in the future. The use of CCR5 inhibitors as a second-line treatment increases the possibility that these reagents will select for more pathogenic CXCR4-using variants. The development of effective CXCR4 antagonists for dual treatment would be beneficial; however, whether long-term treatment with antagonists of the widely expressed CXCR4 receptor is feasible without toxicity is unknown. This review discusses the current status of CCR5 antagonists, their modes of action and their development for therapeutic use.