The study of European integration tends to focus on formal aspects of the integration process: formal decision-making procedures, the role and functioning of institutions, the provisions contained in the treaties, the operation of regulatory regimes in the various policy areas. This is hardly surprising: what is distinctive about the integration process in Europe – what distinguishes integration in Europe from international cooperation in other parts of the world – is the creation and growth of a unique institutional and legal framework structuring the relations between the participating states.1 An early and still important contribution to the understanding of the integration process has been the ‘integration through law’ school which regarded the development of a supranational legal order as the key factor in the explanation of the integration process (Cappelletti et al. 1986). If the formalization of interstate relations is regarded as the essence of the integration process, it is hardly surprising that scholars should concentrate on the formal procedures and the institutionalized arenas for decision-making. However, as has become increasingly evident in the course of recent developments in the European Union, there is an important undercurrent to the formal integration process. This concerns the operation of informal networks which link policy-makers to client groups as well as actors across EU, national and sub-national institutions, and influence (or at least seek to influence) decision-making in the EU. This practice of informal governance is, of course, not a recent phenomenon, but a long-standing dimension of EU politics. This is a recognition that has been reflected by some approaches to European integration: in particular the literature on policy networks and the new institutionalism writing of the 1990s have picked up on the less formal aspects of the EU policy process.2 Nevertheless, informal governance has never been systematically studied and assessed: this is the void that this volume seeks to fill.3