In the European Union, with the coming into force of the Treaty in 1993, police cooperation gained a solid position on the European political agenda. Since then, the EU Council has adopted a significant number of policy instruments aimed at enhancing police cooperation between the EU Member States. But what has been the effect of these instruments on the practices of police cooperation, and how can differences in effect be explained? These two questions guide this doctoral study that investigates 70 instruments adopted by the EU Council between 1995 and 2004. Data for the field research was collected from numerous Council documents and interviews with police practitioners, and was analyzed using both quantitative and qualitative techniques. The book presents the findings of the study, starting with an examination of key concepts related to police cooperation in Europe. It goes on to discuss academic literature on police cooperation, to give an overview of relevant developments in police cooperation practice in Europe and a detailed description of EU Council policy-making on police cooperation. The findings of the field research indicate that, in the shaping of EU Council policy instruments aimed at enhancing police cooperation, it is important to elevate substance (professional rationality) over form (legal nature). Dissertation.