The present volume presents new 2020 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 volume reports 2020 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. The 2020 panel data collections occurred during the start of the COVID-19 pandemic (from March 2020 through November 2020), and this volume constitutes one of the first considerations of possible pandemic effects on prevalence and trends of substance use among the MTF young and middle-aged adults. This volume reports the results of the repeated cross-sectional surveys of all high school graduating classes since 1976 as they are followed 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, called 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, 1975-2020. Volume I, Secondary School Students," see ED615087. For the report from the previous year, see ED608266.]