This paper describes the design of a study to follow the development of boys and girls in secondary schools from the age of 12 through 17 on an annual basis, in order to acquire more information concerning the growth and development of teenagers. In this study, both physical and psychological characteristics are measured. Normal daily diets, usual physical activity, and attitudes towards physical education are measured to assess their influence on physical and psychological characteristics. In view of the inadequacies of pure-longitudinal and of cross-sectional designs, a multiple longitudinal design has been chosen in which four repeated measurements are made in two overlapping cohorts by which age-, time of measurement-, and cohort-effects can be distinguished. Test effects are isolated by comparing the data from the test cohorts with data from an independent sample of identical cohorts from a second "control" school.