This chapter introduces the main concepts used in this collective volume and underlines the importance of international interdependencies for social policy development and highlights how, in recent years, social policy research has undergone far-reaching changes of which perhaps the most important concerns the scope of research, which is no longer exclusively focused on the nation state. Although the welfare state as such is a European “invention” and can be considered a child of the nation state and the Industrial Revolution, its development has also been driven by various inter- and transnational influences. These are at the centre of the contributions.