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This article reviews the 10-year history of tutor development based on the advanced computer tutoring theory (J. R. Anderson, 1983, 1993). We developed production system models in ACT of how students solved problems in LISP, geometry, and algebra. Computer tutors were developed around these cognitive models. Construction of these tutors was guided by a set of eight principles loosely based on the ACT theory. Early evaluations of these tutors usually, but not always, showed significant achievement gains. Best case evaluations showed that students could achieve at least the same level of proficiency as conventional instruction in one third of the time. Empirical studies showed that students were learning skills in production-rule units and that the best tutorial interaction style was one in which the tutor provides immediate feedback, consisting of short and directed error messages. The tutors appear to work …