This study assesses the changing lifestyles, values, and preferences of American youth on a continuing basis. Each year since 1975, at least 13,000 seniors have participated in the annual survey, which is conducted in some 130 high schools nationwide. Since 1991, the study's annual surveys also have included surveys of similar nationally representative samples of eighth- and tenth-grade students. In addition, subsamples of high school seniors from previously participating classes receive follow-up questionnaires by mail each year. This report presents the objectives and theoretical Foundation of the Monitoring the Future Study. [For the designs and the procedures for the Monitoring the Future Study, see "The Monitoring the Future Project after Four Decades: Design and Procedures. Monitoring the Future Occasional Paper Series. Paper 82," ED578425.]