Europeanization as a political process may be understood in terms of both the creation of a European polity and the adaptation of national polities to European integration. This paper examines a key aspect of the interrelationship between these two dimensions of Europeanization. Against expectations, the national politico-administrative systems of European Union member states have not converged. Drawing on a new institutionalist analytical framework, it is shown that these differential patterns of national adaptation correspond to more general models of incremental institutional change and, as such, are likely to be a persisting feature of European integration. This, in turn, leads to a reconceptualization of European governance, which emphasizes the importance of national diversity. The influence of differing national politico-administrative models on the development of the European Union is discussed. The implications of national diversity for policy implementation frameworks are also analysed.