The Centers for Disease Control and Prevention’s (CDC’s) National HIV Behavioral Surveillance (NHBS) serves as a key component of its high-impact prevention approach to reducing the spread of HIV in the United States. It provides data for monitoring behaviors among populations at risk of acquiring or transmitting HIV infection and identifying the populations for whom scientifically proven, cost-effective, scalable interventions are most appropriate. People who inject drugs (PWID) are one of three priority populations. This report summarizes findings from the fourth NHBS data collection among PWID, which was conducted in 2015. This report describes data from 10,485 PWID who participated in NHBS in 2015.