The present volume presents new 2019 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. This report includes 2019 prevalence estimates on numerous illicit and licit substances, examines how substance use differs across this age span, and shows how substance use and related behaviors and attitudes have changed over the past four decades. MTF, now in its 46th 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). Note that the data reported in this volume were collected before the COVID-19 pandemic. The authors report the results of the repeated cross-sectional surveys of all high school graduating classes since 1976 and 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) same-aged youth who also are graduates from high school but not attending college full time, sometimes in the past 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 we refer as the "young adult" sample; and (4) high school graduates at the specific later modal ages of 35, 40, 45, 50, 55, and 60. This volume emphasizes historical and developmental changes in substance use and related attitudes and beliefs occurring at these age strata. [For "Monitoring the Future National Survey Results on Drug Use, 1975-2019. Volume I, Secondary School Students," see ED608265. For the report from the previous year, see ED599071.]