This chapter reviews comparative research on young people’s transitions between initial education and work, focusing on countries belonging to the Organisation for Economic Co-operation and Development (OECD) and especially on European countries. The processes and outcomes of transition vary across countries. Researchers attribute these variations, and their persistence in the face of globalization, to institutional differences between national “transition systems.” The chapter describes four explanatory frameworks that respectively analyze transition systems in terms of characteristics of education systems, labor-market structures, linkages between education and work, and welfare regimes. It reviews typologies of transition systems derived from these explanatory frameworks and their interconnections, and it notes that no typology explains all the variation in national transition systems. It briefly reviews strategies for policy learning from cross-national comparisons of transitions. It concludes that transition systems should be understood as clusters of institutional arrangements that generate a distinctive “logic” of transitions in each country.