Despite the common sense view that small classes are likely to lead to a better quality of teaching and learning, research evidence is not always clear, and has proved contentious. This chapter will argue that research has concentrated on two "generations" of research: first, effects on pupil academic outcomes, and, second, a more recent interest in effects on classroom processes. It will then argue for a third generation of research which tests effective teaching and pedagogies in small classes. Throughout the chapter there is attention to methodological issues and suggested areas for further research. A final section will critically review theories of class size effects, covering ecological and social psychological approaches, and address implications for policy and practice.