More than 15 antiretroviral drugs are now available for clinical use, and have led to signi®cant reductions in morbidity and mortality for HIV infected individuals. Nevertheless, antiviral drug resistance emerges to all these drugs, which limits their bene®t. This review addresses the biological basis of antiretroviral drug resistance, and the prevalence of speci®c drug resistance associated mutations in patients treated with the three currently available classes of agents, namely nucleoside analogue reverse transcriptase inhibitors, non nucleoside reverse transcriptase inhibitors and protease inhibitors. In addition, data on prevalence of HIV drug resistance in untreated individuals published to date are summarised, and the implications of potential transmission of drug resistant HIV is discussed.

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