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 broad 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 populations under study here includes high school graduates in the general population ages 21-30, surveyed annually since 2004; age 35, surveyed annually since 2008; and age 40, surveyed annually since 2010. The present monograph addresses some of the factors that may be preventing further progress against HIV/AIDS, including key behaviors related to the spread of HIV/AIDS in the United States, some of which also relate to the spread of other sexually transmitted diseases (STD). In 2016, about 40,000 individuals became newly infected with HIV in the United States (CDC, 2018a). 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 2018 monographs, see ED589762 (Overview of Key Findings), ED589763 (Volume 1), and ED589764 (Volume 2). For "Monitoring the Future National Survey Results: HIV/AIDS Risk & Protective Behaviors among Adults Ages 21 to 40 in the U.S., 2004-2016," see ED578601.]