Combination antiretroviral therapy (ART) has dramatically improved survival in individuals with human immunodeficiency virus type1 (HIV-1) infection. Despite recent development of more potent regimens with fewer toxicities and lower pill burden, there remains a subpopulation of subjects who fail to achieve and maintain viral suppression while on treatment. Factors known to contribute to virologic failure include suboptimal adherence, drug resistance, suboptimal regimen potency, sequential introduction of single drugs to a failing regimen, and reduced ART exposure due to impaired drug absorption or pharmacokinetic drug-drug interactions.