Since the 1990s, historical institutionalism has established itself as a frequently used approach in the study of European integration. One basic tenet of those who use this approach is to take history seriously in the study of European integration—in particular how historical choices on institutionalizing particular procedures and policies explain subsequent patterns of agency. Looking at the manner in which time and institutional structures affect outcomes is central in this approach. In the context of the European Union (EU), the works that have adopted this approach have typically examined developments in policies and institutions over time. While sharing with other institutionalist approaches (such as rational choice and sociological institutionalism) the recognition that “institutions matter,” historical institutionalism introduced particular concepts such as “path dependence” and “critical juncture” into the study of the EU. The distinct contribution here is the capacity of historical institutionalism to explain the persistence of institutional structures and the continuity of policies as well as the reasons for change. In the study of European integration, this approach has been adopted in many areas of research, ranging from studies about the legal foundations of the EU, the workings within institutions of the EU, the process of enlargement, to analyses of various sectors of EU policy-making, and the study of the multiple crises confronting the integration project in the 2010s.