The use of classroom observation is explored in several capacities. Specific observation instruments that were developed to evaluate the effectiveness of the National Follow Through Program were later used (sometimes in adapted forms) to study early childhood programs, secondary school programs, student teacher effectiveness; and use of time across a school district. Project Follow Through was intended to provide a program analogous to Head Start for economically disadvantaged children over a longer period of time. This chapter presents the fourth and most comprehensive report of Follow Through classroom observation data collected in spring 1973 from 36 sites representing 7 sponsors and 7 program models (35 first grades and 36 third grades). Other studies using the developed Classroom Observation Instrument (COI) in the following are briefly outlined: (1) early childhood education; (2) English as a Second Language; (3) secondary school; (4) staff development; (5) effective use of time training; (6) student evaluation; and (7) student teaching. The observation techniques have provided a means to identify effective instructional practices in a wide range of classroom settings. Two tables and one figure illustrate the discussion. (SLD)