Monitoring the Future (MTF) is a long-term study of American adolescents, college students, and adult high school graduates through age 55. The study is supported under a series of investigatorinitiated, 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 abuse, related to the spread of the human immunodeficiency virus (HIV) responsible for the acquired immunodeficiency syndrome (AIDS). The population under study 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. In recent years, about 45,000 to 50,000 individuals become newly infected in the United States (CDC, 2015a, 2015b). MTF surveys assess both sexual risk behaviors and injection drug use, which are two main sources of HIV infection. In addition to the particular risk of HIV, young adults are at high risk of contracting other sexually transmitted diseases and infections (STDs/STIs). Over half of the 20 million STDs occurring annually in the United States affect individuals aged 15 to 24 (CDC, 2015b; Weinstock et al., 2004). In this monograph, we track some of the key behaviors related to the spread of HIV/AIDS in the United States, some of which also affect the spread of other STDs. The present volume is the fourth monograph in the annual MTF series of reports, all available online from the MTF website. The first monograph, Overview of Key Findings, is published near the beginning of each year and provides early findings on the levels and trends in use of various substances by the nation's 8th-, 10th-, and 12th-grade students surveyed in the previous year (Johnston et al., 2017). "Volume I," available at the beginning of June, provides more detailed and complete findings on the same population (Miech et al., 2017). "Volume II," available at the beginning of August, provides similar prevalence and trend information on the substance-using behaviors of adult high school graduates through age 55, based on a series of follow-up mailed surveys of representative samples of students from each high school graduating class (Schulenberg et al., 2017). "Volume II" has provided findings specific to college students since 1980. HIV/AIDS risk and protective behaviors were introduced into the MTF follow-up surveys in 2004 and findings based on these measures were reported in "Volume II" from 2004 through 2008, after which they were published in separate volumes including the present one. (Individual chapters provide references.)