European institutions are either loathed or underestimated. This book analyses the role of EU institutions in the area of European internal security. From Justice and Home Affairs, this area has become more like a European Area of Freedom, Security and Justice. In this process, the European Commission has demonstrably played the role of an `engine of integration' in areas such a counter-terrorism, policing, asylum, migration and border management. This book uses the framework of supranational policy entrepeuners (SPE), those who stand at the policy window in order to propose, lobby for and sell `their' policy proposal, and synthesises it with insights from the literature on norm entrepreneurship. This book will be of great interest to scholars and practitioners interested in European internal security, European integration, terrorism, security studies and international relations.