It is well known that socio-economic status (SES) at both the student and school level is associated with educational outcomes. Students from higher social backgrounds, on average, have greater educational outcomes than their less privileged peers. Also, a student that attends a school that enrols students from primarily high social backgrounds will, on average, have greater educational outcomes than if she attended a school that enrolled students from low social backgrounds. School SES is related to student outcomes through a complex array of factors, including the quality of curriculum and instruction, resources and the learning environment. This article reviews the research literature about the relationship between school socio-economic composition and student outcomes, the mechanisms by which the relationship is enacted in schools, as well as the structural features of education systems that influence how students from varying social backgrounds are distributed across schools. I conclude with suggestions for future research.