Despite the availability and proven efficacy of medical treatment, many individuals living with HIV in the United States today are not engaged in regular HIV medical care or receiving antiretroviral medications. This journal supplement highlights results of a national 5-year multisite Outreach Initiative, funded by the Health Resources and Services Administration (HRSA) in 2001 to "engage people in HIV care, turn sporadic users of care into regular users, and promote retention in care." The introductory paper for the supplement provides background information on the characteristics of individuals who are not engaged in regular HIV care, the barriers they face, intervention options, and the public policy implications of this issue. Interventions to engage and retain underserved populations living with HIV in medical care are essential to ensure access to medical care and to reduce disparities in health outcomes.