As one cornerstone of European integration, administrative integration denotes how national bureaucracies and the EU administrative apparatus increasingly intermesh, interact and ultimately integrate. The overall rationale of this study is to reveal the basic features of this phenomenon and to account theoretically and empirically for administrative integration across the EU - nation- state intersection. The current study raises two main questions: (i) How well integrated are the national central administrations of European nation- states and the administrative apparatus of the European Union (EU)? And, (ii) how should we account for processes whereby national systems and the EU administrative apparatus become increasingly intertwined, intermeshed, and interlinked?