This study departed from one of the most central points of contention in the study of European governance: are the EU’s supranational institutions engines of integration that actively drive the process of integration, or simply obedient servants that passively fulfil the technical functions they have been delegated by member governments? At the heart of this debate are competing claims about the degree of independent causal influence exerted by the Commission, the ECJ, and the EP on the course of European integration. As opposed to existing research, which is heavily focused on the agenda-setting phase of EU policy-making, this book has examined the influence of the Commission and the ECJ in post-decisional compliance politics. EU decisionmaking is of little value per se, and does not result in actual policy results, unless member state compliance with EU rules can be secured. The extent to which the Commission and the ECJ can enforce compliance more strenuously than desired by member governments and originally intended is therefore pivotal to the understanding of European governance.