A review of evaluative research on the Keller plan establishes the following points: 1) The Keller plan is an attractive teaching method to most students. In every published report, students rate the Keller plan much more favorably than teaching by lecture. 2) Self-pacing and interaction with tutors seem to be the features of the Keller courses most favored by students. 3) Several investigators report higher-than-average withdrawal rates for their Keller sections. The conditions that influence withdrawal and procrastination in Keller courses have been studied, and it seems possible to control procrastination and withdrawal through course design. 4) Content learning (as measured by final examinations) is adequate in Keller courses. In the published studies, final examination performance in Keller sections always equals, and usually exceeds, performance in lecture sessions. 5) Students almost invariably report that they learn more in PSI than in lecture courses, and also nearly always report putting more time and effort into the Keller courses.