Antiretroviral compounds can select viral strains presenting mutations of the HIV genome. Certain genotypic modifications are expressed by phenotypic resistance. There is no cross resistance between different classes of compounds (nucleosides, non nucleosides, antiproteases), but cross resistance is common within a given therapeutic class. HIV resistance to antiretroviral compounds is one of the principal causes of failure of antiretroviral treatments but cannot explain all escapes. The number of resistance mutations is higher in patients with high viral loads and in patients on multiple drug regimens. Currently resistance testing is limited to clinical research protocols. The usefulness of resistance testing remains to be validated. However the most eminent indications are epidemiological surveillance of primary resistance in primary infections, therapeutic adaptation after accidental exposure to HIV, and management of seropositive pregnant women. Recent retrospective studies have shown that the genotype and the phenotype after a first line treatment failure predict response to certain therapeutic combinations. In the near future, resistance testing could be useful to adapt antiviral strategies after earlier treatment failure.