The purpose of this paper is to provide a detailed description of the Monitoring the Future research design, including sampling, data collection procedures, measurement content, and questionnaire format. Monitoring the Future is designed to assess the changing lifestyles, values, and preferences of American youth on a continuing basis. Since the project started there has been a dramatic change in attitudes and behaviors that the project has been able to monitor, particularly concerning drug use. One important reason is that the basic study design described in the initial paper has remained constant in its fundamental characteristics and this consistency in survey methods is seen as a key condition for successfully measuring change. From the outset, the project was designed with two interrelated components, an annual nationwide survey of high school seniors, and a periodic follow-up questionnaire mailed to subsamples of each class cohort. The project has monitored four kinds of trends: (1) changes common to all cohorts in a given historical period; (2) maturational changes or age effects which show up consistently in the longitudinal data; (3) changes from one graduating class to another; and (4) longitudinal changes reflecting the differential impacts of important post-high school environments and major role transitions. (Contains 96 references and 17 appendixes.) (JDM)