This chapter reviews international comparative studies on the determinants of socioeconomic inequality in student performance. We were interested in studies of explanatory variables that are amenable to educational policy interventions. To identify such publications, we developed a comprehensive search strategy and conducted an electronic search based on six databases. We also manually searched two existing hand-picked reviews. After duplicates were removed, the search resulted in 814 references, of which a total of 35 studies met the eligibility criteria. The included studies investigated diverse topics such as learning environments inside and outside of school, educational expenditure, teacher education, autonomy, accountability, differentiation, and competition from private schools. Most studies are descriptive in nature and their findings are sometimes ambiguous. Despite these limitations, we tentatively conclude that the opportunity of choice reinforces inequality. Measures that target social selection can be effective.