This chapter studies causation, which occupies a central place in the social sciences. In their attempts to understand and explain ‘why’ social, economic, and political phenomena occur, scholars have dealt with causality in many different ways. The way to define and observe causal relationships has always been at the heart of harsh academic debates in social as well as natural sciences. Drawing on distinctive ontological and epistemological standpoints, at least four different understandings of causation have emerged in political science. Most authors have adopted a correlational-probabilistic understanding of causation, but some have preferred a configurational one, while others have adopted a mechanistic or even a counterfactual understanding. To illustrate the concrete methodological challenges generated by this theoretical pluralism, the chapter discusses how scholars have dealt with causality to explain the impact of European integration on domestic policies and institutions.