Monitoring the Future (MTF) is a long-term study of American adolescents, college students, and adult high school graduates through age 60. The study is supported under a series of investigator-initiated, competing research grants from the National Institute on Drug Abuse and has been conducted annually by the University of Michigan's Institute for Social Research since 1975. The present monograph focuses on a range of behaviors, including certain forms of substance use, related to the spread of the human immunodeficiency virus (HIV), which is responsible for acquired immunodeficiency syndrome (AIDS). The ages covered in this study contain the two age bands with the highest rates of newly diagnosed HIV infection in the United States: namely, ages 20-24 and 25-29. High school graduates who fall into this age range each year have been surveyed annually since 2004. This monograph tracks key behaviors related to the spread of HIV/AIDS in the United States. In 2016, about 40,000 individuals became newly infected with HIV in the United States (CDC, 2017). MTF surveys assess both sexual risk behaviors and injection drug use (including needle sharing), which are two main sources of HIV infection. The present volume is the fourth monograph in the MTF series of annual reports. [For the first three 2019 monographs, see ED594190 (Overview of Key Findings), ED599067 (Volume 1), and ED599071 (Volume 2). For "Monitoring the Future National Survey Results: HIV/AIDS Risk & Protective Behaviors among Adults Ages 21 to 40 in the U.S., 2004-2017," see ED611904.]