Abstract. This thematic review offers a conceptual and analytical framework to assess, identify, and explain the processes of dualization in labor markets and social protection systems in 21st-century welfare states. After a brief historical examination of the concept's origins, we review the primary definitions, theories, and typologies developed to explore and measure dualization, connecting the fields of social policy and political economy. Our systematic review reveals that dualization varies significantly across welfare regimes. Following an institutionalist approach, we identify four key institutional factors that shape the differential impact and scope of dualization: trade unions' strategies, governmental partisanship, coalitions of interests, and the growing influence of international organizations, particularly the EU. Our findings demonstrate that these factors interact through complex direct and indirect mechanisms, mediated by electoral considerations, power resources, institutional constraints, and political opportunities. The review acknowledges the complexities and ambiguities surrounding the notion of dualization. We highlight the importance of institutional and policy responses at national and international levels to address dualization processes, particularly in light of recent crises like the COVID-19 pandemic