Information on patterns of resistance to and cross-resistance between antiretroviral agents is increasingly available and may be important for decisions on how to combine drugs to achieve an optimum antiviral effect in future therapeutic strategies. The increasing number of heavily pre-treated patients have changed the approach to assist therapeutic decision-making in patients failing therapy. Recent studies give us a more comprehensive analysis of the link of such mutations with a rebound in viral load and appearance of drug resistance. However, not all sites of mutations are known and the effects of interactions between them at different codons has not yet been well understood. Moreover, the interpretation is made from generalisations based on drug testing in vitro and there are scarce clinical data available.This work tries to summarize the most important studies to date, in order to know the significance of drug resistance as well as their potential use in everyday clinical practice.