Timely and optimal HIV primary care is a key tenet of the Ryan White CARE Act, a safety net programme for vulnerable and marginalized people living with HIV in the USA. Health services researchers, local providers and policy makers suspect that ancillary services are necessary to improve entry into and retention in HIV primary care for vulnerable populations experiencing barriers to HIV services, including access to antiretroviral therapies. This paper provides background to the eight studies featured in this special supplement to AIDS Care. The eight studies examine retrospectively ancillary (support) services data collected after 1996 in six HIV epicenters (New York and Chicago, plus four sites included in the Client Demonstration project-Los Angeles, San Francisco, Orange County [California] and Washington, DC), three smaller hard-hit cities (Boston, New Orleans and St Louis) and several states (California, plus Michigan and Virginia from the Client Demonstration Projects). These varied delivery settings serve racial and ethnic minority populations, men who have sex with men, injection drug users, women and mothers. The studies use a range of analytic approaches to understand whether receipt of certain enabling services correlated with early entry into and retention in care. Ancillary services (support services such as case management, housing, food, transportation, mental health and substance abuse treatment) are used by local HIV medical and community-based organizations in facilitative strategies directed to populations that have difficulty entering or staying in HIV primary care. Understanding the contribution of ancillary services to timely entry into and consistent use of primary care, including the expanding range of HIV therapeutics, is important to service delivery system planners and resource allocation decision-makers.