This paper uses a case history approach to present the evolution of two longitudinal studies, Youth in Transition, which involved 8 years of data collection (1966-1974) and Monitoring the Future, for which data collection began in 1975 and continues in the present. The Youth in Transition project is described as a study of the causes and consequences of dropping out of high school. Five key decisions about research design made during the program planning phase are discussed. Expansions of the project over an 8-year period are explained. It is noted that the Youth in Transition study laid the groundwork for the Monitorig the Future project and that the studies share similarities in content and methodology. Eight lessons which were learned from the Youth in Transition study and which affected plans for the Monitoring the Future project are listed. The Monitoring the Future project is described as an attempt to extend many of the trend analyses from the first study. Funding, initial data collection from 16,000 seniors in 125 high schools, and follow-up data collections are briefly presented. It is noted that one important product of this project has been extensive descriptive reporting of drug use levels and trends among high school seniors. A continuation of the collection, analysis, and reporting of these data is recommended, especially since the lengthening span of time makes the analyses richer with each new year of data collection. (NB)