This paper summarizes the results of fourteen separate studies comparing the learning outcomes of a new instructional procedure, the ''Personalized System of Instruction'' (PSI), with the learning outcomes of conventional approaches to college teaching. The major conclusion suggested by this summary of research is that, when evaluated by average student performance on course content examinations, PSI has proven superior to the conventional methods with which it has experimentally been compared. The author examines features of PSI courses which probably account for this superiority, and he discusses some of the theoretical and practical implications of research on PSI conducted between 1967 and 1974.