This article discusses key findings from eduLIFE, a cross-national project that examined the emergence of social inequalities in 17 countries characterized by different models of secondary education. First, we build upon existing international studies and propose a broader classification of forms of differentiation in secondary education. Second, we elaborate a fourfold typology of secondary education systems. Third, we provide a longitudinal and comparative analysis of how social background, academic performance, and forms of secondary schooling create heterogeneous educational opportunities for recent generations. In particular, we discuss: (1) the allocation of students to different forms of secondary schooling; (2) student mobility among different types of education; and (3) the consequences of differentiation in secondary schooling for students’ educational careers. Our findings suggest that, on average, more privileged families successfully exploit the opportunities provided by specific institutional configurations of school systems in order to secure the most favourable outcomes for their children.