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 funded 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 population under study here includes high school graduates in the general population, ages 21-30. High school graduates who fall into this age range each year have been surveyed annually since 2004. HIV infection is clearly a serious public health concern. Worldwide, about 36.9 million people were living with AIDS at the end of 2017 (UNAIDS, 2018). In the United States, about 1.1 million people were living with diagnosed HIV infection as of 2019 (CDC, 2021), and 1 in 7 were unaware of their infection (CDC, 2021). The present monograph addresses some of the factors that may have been preventing greater progress against HIV/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. This monograph tracks key behaviors related to the spread of HIV/AIDS in the United States. In 2019, over 36,000 individuals became newly infected with HIV in the United States (CDC, 2021). 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 published this year in the annual series of reports. [For "Monitoring the Future National Survey Results: HIV/AIDS Risk & Protective Behaviors among Adults Ages 21 to 30 in the U.S., 2004-2019," see ED611887.]