Understanding resistance to antiretroviral therapy plays an ever more crucial role in managing HIV infection as new agents - including several in new antiretroviral classes - promise better control of multidrug-resistant virus in the developed world. Yet these new drugs have different, and often complex, resistance profiles. At the same time, resistance has assumed a key role in developing countries as access to additional antiretrovirals expands in the face of first-line regimen failures. Every year the International HIV Drug Resistance Workshop gathers leading investigators and resistance-savvy clinicians to share unpublished, peer-reviewed research on the mechanisms, pathogenesis, epidemiology, and clinical implications of resistance to licensed and experimental antivirals. The 2007 workshop, held on 12-16 June, proved particularly notable for its exploration of resistance to two new antiretroviral classes, integrase inhibitors and CCR5 antagonists, as well as to agents that control hepatitis C virus (HCV) infection. This report summarizes most oral presentations from the workshop and many posters.