This paper utilizes case study findings of the implementation of educational computing in two schools, one elementary school and one fifth- through sixth-grade school, to reflect on recurrent patterns that account for the slow pace of change in instruction. In particular the study focused on the structural arrangements of teachers' work and the way in which teachers made sense of their world. The administration at both schools aimed its support of instructional computing at providing in-service training for teachers and purchasing hardware and software, neither of which is enough to create conditions for extensive and systematic instructional computing in the classrooms. A computer literate staff alone does not produce generalized computer use. Command over technical and social resources is a necessary but not sufficient condition for the systematic implementation of instructional computer activities. In the sites studied, one of the strongest determinants of how extensively and in which ways computers were used was the structure of the schools. Another strong influence on educational computing was the staff's occupational culture, their perceptions of the student body and what constitutes legitimate school knowledge. The analysis is based on an ethnographic study conducted in the 1984-85 school year, 3 to 4 years after computers had been introduced in these schools. (Contains 26 references.) (Author/JB)