Highly active antiretroviral therapy (HAART) refers to a broad category of treatment regimens usually comprised of three or more antiretroviral drugs that, in previously untreated HIV-1-infected patients, are expected to reduce plasma virus levels below the limits of detection. Most HAART regimens include drugs from at least two of the three classes of antiretroviral therapy (nucleoside analog reverse transcriptase (RT) inhibitors, non-nucleoside analog RT inhibitors, and protease inhibitors). In deciding when to initiate antiretroviral therapy, physicians and their patients must balance the virological and immunological benefits of early treatment with the costs of drug therapy, the risk of drug side effects, and the risk of drug resistance if adherence is suboptimal. In previously untreated patients, HIV-1 replication can be suppressed indefinitely with certain HAART regimens. In previously treated patients, the benefits of HAART are often significantly diminished.