The optimal time to start antiretroviral therapy (ART) for human immunodeficiency virus (HIV)-infected individuals remains uncertain. Although current ART regimens are effective in suppressing viremia and enhancing immune function and are increasingly convenient and well tolerated, ongoing concerns remain about adherence, drug-related toxicities, drug resistance, and cost. Although few clinical trials results are currently available to inform the question of when to start ART, large clinical cohorts clearly have demonstrated the benefits of earlier initiation of ART for reducing both HIV-related and non-HIV-related clinical events. Additional data suggest that the strategy of earlier initiation of ART is cost-effective and efficient. Consequently, many antiretroviral guidelines from around the world now recommend routine initiation of ART when the CD4 cell count decreases to <350 cells/microL or at higher CD4 cell counts for certain subgroups of HIV-infected individuals, such as pregnant and/or breast-feeding women and persons with HIV-related nephropathy or hepatitis virus coinfection. Additional cohort and clinical trials data are needed.