Recently developed techniques of parallel emulsion sequencing of thousands of amplicon clones (Ultra deep Sequencing (UDS), Roche Diagnostics/454 Life Sciences) increase the sensibility to detect polymorphisms and mutations in highly variable genomes such as in HIV-1 in several orders of magnitude. In addition, this technique allows stablishing genetic linkage of such mutations and polymorphisms in thousands of HIV-1 clonal sequences for every patient and point of follow-up. This generates an unprecedented opportunity to characterize the nature of HIV-1 variability and the physiopathology of antiretroviral resistance in depth. Ultrasensitive resistance tests hold the promise of improving the clinical management of HIV-1 seropositive patients, avoiding unnecessary toxicities, improving epidemiological estimations of antiretroviral resistance, improving the knowledge of the pathogenesis of HIV-1 infection and antiretroviral resistance, and improving the cost-efficiency of HIV-related pharmaceutical cost.