This contribution introduces the concept of ‘activism’ to the study of EU supranational institutions and their role in EU policy making and European integration. While long present in studies of the European Court of Justice, ‘activism’ has rarely been examined systematically in the context of analyses of other supranational institutions. This contribution offers a definition of ‘supranational institutional activism’, examines its analytical usefulness in relation to other concepts such as ‘entrepreneurship’ and through the lens of a number of political science and EU integration theories and analytical approaches. The specific analytical insights derived from the disciplines of political science, history and legal scholarship of the twelve articles of this special edition on ‘supranational institutional activism’ are also considered. While the powers and roles of EU supranational institutions have been examined in numerous studies, this article presents a concept that can contribute to a more systematic and comprehensive understanding of the contribution of these entities to EU policy making and European integration.