The “World Politics of Social Investment” (WoPSI) project aims at explaining variance in social investment agendas and social in-vestment reforms across democratic countries in different regions of the World. Virtually all capitalist economies grapple with challenges of demographic change, slow economic growth, poor employment performance and increasing poverty rates. In dealing with these pro-blems, a social investment strategy appeals to a wide audience, both political and academic. However, social investment reforms and per-formances in democratic countries around the globe are highly une-qual and remain fragmentary: different countries have implemented different types of policy instruments, with different functions, at diffe-rent points in time, and to different degrees. Despite a growing number of scientific contributions on social in-vestment reforms and their effects, a systematic mapping of the de-sign of social investment agendas and policies in different democra-tic countries around the globe is still lacking. Moreover, we lack an explanation for the variance in the development of social investment policy reforms. This is where we locate the research interest of the project presented in this paper. In order to understand why social investment agendas and policies have developed differently across countries, we need to study the politics of social investment reforms. Thus, we ask: How do social investment agendas and social in-vestment policy reforms vary across democratic countries around the globe? Under what political conditions do social investment agendas and/or reforms develop ? In this paper, we situate these questions in the existing state of the literature, and we outline a way to answer these research questions in the context of social policy reforms in Latin America, East Asia, as well as Western and Eastern Europe. We argue that political coali-tions (actors and their interests), as well as the institutional embed-dedness of social investment politics are key factors in explaining the high variety of social investment agendas and policies between countries.