Street outreach programs for human immunodeficiency virus (HIV) prevention are designed to deliver HIV prevention messages, materials, and referral services to high-risk persons outside of traditional health-care and drug-treatment clinics. The Acquired Immunodeficiency Syndrome (AIDS) Evaluation of Street Outreach Projects (AESOP) is an eight-site study designed by CDC in collaboration with researchers in each of the sites to better understand client characteristics, service delivery, and the impact of street outreach programs on the risk behaviors of high-risk populations. The populations studied are injecting-drug users (IDUs) in five of the eight sites and youth in high-risk situations (YHRS) (i.e., youths aged 12-23 years who are homeless or runaway or who support themselves through the "street economy" of drugs, prostitution, pan-handling, and crime) in three sites. This report summarizes information collected during the first 2 years of the project.