The development of a contextual classroom observation system is described. The system consists of an instrument, trained observers, data handling, and analysis. Thus, instrument development was necessarily paralleled by the development of training procedures, data recording and processing systems, programming, and analytic techniques. The comprehensive observation system described was initially developed for use in the evaluation of Head Start and Follow Through programs, but has received wide application and has led to the development of a number of derivative observation systems. Data collection forms concerning the physical classroom environment, the snapshot of the classroom, and five-minute samples of interpersonal relations are included. (Author/GDC)