In May 2000, the HIV/AIDS Bureau of the Health Resources and Services Administration convened HIV experts from throughout the country to identify new and emerging areas of research needed to guide policy and programmatic decisions on HIV service delivery to vulnerable populations. This article describes the process used to develop an evaluation/research agenda, discusses key findings and recommendations of the conference, and proposes a set of principles to guide the design and conduct of future investigations. Conference participants identified nine major evaluation/research themes that span the continuum of HIV behavioral prevention services and treatment. They recommended focusing future research on questions relevant to populations experiencing rapid rates of increase in HIV infection (for example, women, people of color, and adolescents and young adults) and considering explanatory factors at multiple levels of analysis (individual, clinician, organization, service delivery system, and environment).