Assays that detect antiretroviral drug resistance in human immunodeficiency virus have recently become available to clinicians. Phenotypic assays measure the drug susceptibility of the virus by determining the concentration of drug that inhibits viral replication in tissue culture. Genotypic assays determine the presence of mutations that are known to confer decreased drug susceptibility. Although each type of assay has specific advantages, limitations associated with these tests often complicate the interpretation of results. Several retrospective clinical trials have suggested that resistance testing may be useful in the assessment of the success of salvage antiretroviral therapy. Prospective, controlled trials have demonstrated that resistance testing improves short-term virological response. Resistance testing is currently recommended to help guide the choice of new drugs for patients after treatment has failed and for pregnant women. Resistance testing should also be considered for treatment-naïve patients, to detect transmission of resistant virus.