For many years, comparative welfare state research has followed a ‘methodological nationalism’ in the sense that countries were treated as independent units. Yet the recent ‘spatial turn’ in comparative politics has also influenced welfare state research. For some years now, the field has been witnessing a growing interest in questions about interdependencies and policy diffusion between countries. In this article, we provide a structured overview of the state of the art in the policy diffusion and transfer literature that deals specifically with social policy. We present and critically evaluate existing theoretical concepts and quantitative and qualitative methodological approaches that enable the analysis of interdependencies between countries. Moreover, we summarize the empirical findings of quantitative and qualitative studies on the diffusion and transfer of social policy, from some pioneering studies to the latest findings. Against this background we point out what we believe to be promising avenues for future research. We focus on five areas: theoretical work on the mechanisms underlying diffusion and transfer; methodological approaches; the impact of domestic institutions and policy characteristics on social policy diffusion and transfer; programme-specific dynamics; and the systematic combination of horizontal and vertical interdependencies.