This book provides an in-depth empirical analysis and theoretical history of the institutional development of EU police cooperation, with a focus on the creation and integration of Europol. Presenting a thoroughly comprehensive analysis, the book systematically traces integration dynamics and the evolution of EU police cooperation over a 40-year period, assessing the influence of cross-country interdependencies, politicisation and policy entrepreneurship on Member States' behaviour and institutional choice. By combining a wealth of sources including previously unpublished sources and personal insights from key decision-makers, it explores which driving factors shape processes of differentiation and integration in this sovereignty-sensitive policy domain, and how, and attempts to explain state preferences on international police cooperation in the light of major theories of European integration. The book will be of key interest to students, scholars and practitioners working in or on the fields of police cooperation, Justice and Home Affairs policy, EU governance and security studies, both at national and European level.