Personalized systems of instruction (PSI), also called individualized instruction or contingency-managed instruction, were developed in the mid-1960s at the college level by Fred Keller, a reinforcement learning theorist. The Keller plan consists of five features: self-pacing, unit mastery, student tutors, optional motivational lectures, and learning from written material. The Keller plan appears to work well because it is consistent with at least ten educational principles for which there is some empirical support, such as active responding and specification of objectives. Keller's plan may be implemented in five stages: assessing entering behaviors, specification of course objectives, selecting resources and activities, establishing the course, and evaluating student performance. Systematic efforts at evaluating PSI are beginning to appear. The two most extensive evaluations show that PSI students perform better on all types of examinations, have longer retention, and have more positive attitudes towards courses than students taking conventional, structure-type courses. (Author/ND)