HIV-related morbidity and mortality have been dramatically improved in populations treated with combination antiretroviral therapy. Although it is widely recognized that adherence to the antiretroviral medication regimens is vital to treatment success, rates of adherence to the regimens are often poor. There is a large body of research exploring the problem of adherence to antiretroviral medications. The literature is, to date, dominated by reports identifying factors that are predictive or associated with antiretroviral adherence. Adherence is increasingly understood as a dynamic behavior influenced by a matrix of interrelated factors that change over time. Preliminary reports suggest varying degrees of success with strategies designed to improve adherence. Multifaceted strategies appear to be the most promising; however, there are few controlled studies substantiating the effectiveness of these approaches and the mechanisms by which the interventions promote adherence are not well understood. More well powered, rigorously evaluated antiretroviral adherence intervention trials are urgently needed. Further, problems in the field exist because of limitations in the available adherence measures. This paper provides a comprehensive review and analysis of the state-of-thescience of this body of work. Despite substantial attention to antiretroviral adherence in recent years, there remain significant gaps in our understanding.