Despite over 30 years of research into the effects of school composition or "mix," there is remarkably little consensus over the nature and size of school compositional and peer effects. Developing an analytical review of international research in this area, this chapter begins by discussing conceptual and methodological problems related to theorising school compositional effects. A model is developed through which the adequacy of existing studies for capturing compositional effects can be judged. This model helps to explain why existing studies have failed to reach consensus since the modelling techniques vary and none approach the adequacy criteria articulated in the model. Qualitative research can go a long way towards illuminating the subtle processes that are likely to underlie school compositional effects. The inherent difficulties in "measuring" elements of compositional effects have left debate around the importance of school compositional effects open to political and ideological considerations, since the outcomes of key policy decisions such as school choice, effectiveness, and accountability depend on the findings.