The acceleration of European integration in the 1990s, essentially through an increase in the scope and frequency of Intergovernmental Conferences, has provided renewed debate between intergovernmental and supranational approaches to the European Union. While efforts towards theory development have significantly increased, the ontology of such work tends to rely on actors and on specific decisions or events rather than on the elements which structure the process. In an attempt to "bring process back" into the study of the integration process, the article distinguishes between three different layers of change in Europe. Among these, policy-making and constitutional reform have received ample attention in the literature, while the deeper layer of change, structural transformation, has largely been neglected. In redressing this balance, the article suggests approaches such as constructivism, historical institutionalism and structuration theory -advances which have been important in other disciplines -to be utilised for the development of a more comprehensive agenda of integration theory development