The emergence of drug resistance is a major factor contributing to the failure of antiretroviral therapy in HIV-infected patients. Drug resistance can be detected by genotypic or phenotypic assays, both having distinct advantages and disadvantages. Results from genotypic and phenotypic testing are helpful in excluding from the subsequent regimen drugs to which the resistance is identified, and both tests predict virologic response to salvage therapy in patients who have failed a previous regimen. Resistance testing is likely to be beneficial as an aid in selecting a salvage regimen.