This study seeks to understand how European integration works. It examines the gradual, unobtrusive changes that happen in the daily flurry of decision-making activity in the European Union (EU). Specifically, it identifies and explains occurrences of change in the EU's police and justice cooperation. The study thereby captures the EU as an evolving polity where - in view of its institutionally dense environment - interests of a multitude of parties interacting on routine basis are more likely to intermesh, rather than compete. On the basis of three case studies, involving discourse analysis of policy documents and interviews with people from the professional field, the study seeks to capture instances where actors under given circumstances engage in deliberative discourse which induces them to shift or change their interests towards a common understanding that provides the basis for a policy outcome. The basic finding is that when arguments carry the day, negotiating parties are less inclined to take recourse to interstate or 'intergovernmental' tactics that would enable them to maximize their own national interests. One of the implications of the study's findings is that we need to re-evaluate the generalisation that the EU is being mould and made only through interstate bargaining politics, even in policy areas where the paradigms of national statehood and sovereignty are well-established (such as justice and home affairs). It also means that the scope for member state action to shape EU policy on an individual basis is quite limited and that it is contingent on a wide variety of conditions operating at the day-to-day level of EU decision making.