The National HIV Behavioral Surveillance system (NHBS) was designed to monitor HIV prevalence and risk factors for infection among higher-risk individuals, i.e., sexually active men who have sex with men who attend venues, injection drug users who injected in the past 12 months, and heterosexuals living in low socioeconomic urban areas. These groups were selected as priorities for behavioral surveillance since they represent the major HIV transmission routes and the populations with the highest HIV burden. NHBS contributes to the nation’s program of HIV surveillance by being the only multi-site population-based system that provides estimates on key HIV prevention measures among high-risk HIV-negative individuals, HIV-positive individuals unaware of their infection, and HIV-positive individuals aware of their infection who are in and out of care. Accurate and precise data on the behaviors in these populations are critical for tracking the epidemic, planning effective responses, and monitoring and evaluating those responses. Reports in this supplement illustrate the uses of NHBS data at the national and local level and reflect ongoing efforts to improve the system and methods. As we look to the future, behavioral surveillance remains essential for characterizing and monitoring the burden of HIV infection and sexual and behavioral risks.