Given a computer-based instructional program and a school in which to work, it is possible to supplement or enrich the teacher's instruction by taking over the more routine daily tasks, presenting special materials, or giving the daily lesson itself. The task of the present project was to prepare a program in arithmetic to review and teach the basic number facts as a supplement to the teacher's daily instruction at the fourth-grade level. In addition, the project was concerned with gaining a clearer picture of the optimum teacher-machine interaction pattern that would take full advantage of both.