Witness for the prosecution: The IAS-USA and Euro-Resistance Group HIV guidelines recommend the use of resistance testing for all patients experiencing treatment failure for whom therapy change is being considered. However, these assays suffer from several limitations (problems in sensitivity, specificity, complexity of interpretation, cost) and the results of the prospective studies evaluating genotype-guided treatment in HIV patients failing antiretroviral treatment are inconclusive and partially contrasting (virological benefit is short-term). On this basis, incorporating genotypic resistance assays into the clinical management of HIV patients experiencing first treatment failure is not a sufficiently evidence-based practice. Witness for the defence: Highly active antiretroviral therapy (HAART) has markedly improved the prognosis of HIV-infected patients by controlling HIV replication. However, HAART fails to control HIV replication in an increasing number of patients as a result of a complex array of causes. There is now substantial evidence that the emergence of drug resistance is a leading cause (as well as consequence) of antiretroviral therapy failure. Moreover, HIV drug resistance can be transmitted and this can favour initial treatment failure. Several retrospective and prospective studies have indicated that both genotypic and phenotypic HIV-1 drug resistance testing results are associated with, or predictive of, the virological outcome. As a consequence, international guidelines have soundly recommended the use of resistance testing to guide treatment choices after virological failure. The rationale and advantages of using such testing after first virological failure will be discussed.