This volume presents new 2018 findings from the U.S. national Monitoring the Future (MTF) follow-up study concerning substance use among the nation's college students and adults from ages 19 through 60. The authors report 2018 prevalence estimates on numerous illicit and licit substances, examine how substance use differs across this age span, and show how substance use and related behaviors and attitudes have changed over the past four decades. MTF, now in its 45th year, is a research program conducted at the University of Michigan's Institute for Social Research under a series of investigator-initiated, competing research grants from the National Institute on Drug Abuse--one of the National Institutes of Health. The integrated MTF study comprises several ongoing series of annual surveys of nationally representative samples of 8th and 10th grade students (begun in 1991), 12th grade students (begun in 1975), and high school graduates followed into adulthood (begun in 1976). The authors report the results of the repeated cross-sectional surveys of all high school graduating classes since 1976 as they follow them into their adult years. Segments of the general adult population represented in these follow-up surveys include: (1) U.S. college students; (2) their age-peers who are not attending college, sometimes called the "forgotten half"; (3) all young adult high school graduates of modal ages 19 to 30 (or 19-28 for trend estimates), to whom the authors refer to as the "young adult" sample; and (4) high school graduates at the specific later modal ages of 35, 40, 45, 50, and 55. This volume emphasizes historical and developmental changes in substance use and related attitudes and beliefs occurring at these age strata.