Substance use is a leading cause of preventable morbidity and mortality; it is in large part why, among 17 high-income nations, people in the U.S. have the highest probability of dying by age 50. Substance use is also an important contributor to many social ills including child and spousal abuse, violence more generally, theft, suicide, and more; and it typically is initiated during adolescence. It warrants our sustained attention. Monitoring the Future (MTF) is designed to give such attention to substance use among the nation's youth and adults. It is an investigator-initiated study that originated with, and is conducted by, a team of research professors at the University of Michigan's Institute for Social Research. Since its onset in 1975, MTF has been funded continuously by the National Institute on Drug Abuse -- one of the National Institutes of Health -- under a series of peer-reviewed, competitive research grants. The 2019 survey, reported here, is the 45th consecutive survey of 12th grade students and the 29th such survey of 8th and 10th graders. Two of the major topics included in the present volume are: (1) the prevalence and frequency of use of a great many substances, both licit and illicit, among U.S. secondary school students in 8th , 10th, and 12th grades; and (2) historical trends in use by students in those grades. Distinctions are made among important demographic subgroups in these populations based on gender, college plans, region of the country, population density, parent education, and race/ethnicity. MTF has demonstrated that key attitudes and beliefs about drug use are important determinants of usage trends, in particular the amount of risk to the user perceived to be associated with the various drugs and disapproval of using them; thus, those measures also are tracked over time, as are students' perceptions of certain relevant aspects of the social environment--in particular, perceived availability, peer norms, use by friends, and exposure to use by others of the various drugs. Data on grade of first use, discontinuation of use, trends in use in lower grades, and intensity of use are also reported here. [For "Monitoring the Future National Survey Results on Drug Use, 1975-2019. Volume II, College Students & Adults Ages 19-60," see ED608266. For the report from the previous year, see ED599067.]