According to Andrew Moravcsik the EU has tried to secure "credible commitments" by pooling and delegating sovereignty to common institutions. The European Commission and the European Court of Justice (ECJ) have what some scholars call supranational powers. An increasing number of decisions in the Council of Ministers can be taken by a qualified majority vote (QMV). According to other scholars, including Walter Mattli, leadership is important to overcome "collective action problems" in international cooperation. This paper will contrast these different explanations of successful regional integration and compare the EU with integration in South America, MERCOSUR in particular. What is the current state of regional integration in South America? To what extent has MERCOSUR created autonomous common institutions? Have some member states been able to provide leadership, and if so, to what effect? The discussion will be put into a comparative perspective, referring also to other integration experiences in other parts of the world.