Throughout the world, school choice is becoming increasingly prevalent. While most of the earlier works on school choice were polemical, empirical studies of the effects on student outcomes are becoming more common. The increase in school choice is also bringing renewed attention to school composition and segregation, and their effects on student outcomes. The effect of school composition on student outcomes has been recognized at least since the 1966 Coleman Report, which found that the social composition of schools had a larger effect on the educational achievement of disadvantaged African-American students than school resources. Policy attempts to manage school composition and minimize school segregation have largely been ineffective in many countries. A new line of research is examining the effects of school composition and school sector (non-government and government) on individual student outcomes, disaggregated by student socio-economic status. In particular, studies are examining whether the effects on student outcomes are the same for all students. A more nuanced view of the effects of school composition on student outcomes may help policy makers and analysts mediate the negative effects of school choice. The paper concludes with other potential lines of research on school composition and student outcomes.