Mastery learning is a philosophically based approach to the design of classroom environments that is currently creating controversy in the educational research and development community. Critics of mastery learning assert that mastery approaches to instruction are rigid, mechanistic, training strategies (Groff, 1974; Jaynes, 1975); that they can only give students the simple skills required to survive in a closed society (Cronbach, 1972); and that they do not appreciate the complexities of school learning (L. S. Bowen, 1975). Adherents of mastery approaches to instruction maintain that they are flexible, humanistic, educational strategies (Levin, 1974; Scriven, 1975); that they can provide students with the complex skills needed to prosper in an increasingly open society (H. M. Levin, 1975); and that they do take into account the realities of classroom life (Block & Anderson, 1975). In this chapter we propose to introduce the basic ideas and practices that have generated this controversy, as well as to review the associated research. We begin the chapter with a broad overview of mastery learning philosophy, theory, and practice. We then take a relatively microscopic view of four types or classes of research generated by these ideas and practices. In the final section we take a more macroscopic view of this research and ask what our findings may mean in practical, theoretical, and ideological terms.