An increasing number of authors describe the European Union as an advanced form of transgovernmentalism. Whether called Europeanization, supranational intergovernmentalism, multilevel governance, administrative fusion or Brusselisation, the transgovernmentalist thesis states that European politics is shaped by the growing interaction of national government officials at every level of the decision-making process. This paper tests the transgovernmentalist thesis by looking at patterns of formal and informal cooperation in the framework of the European Security and Defence Policy (ESDP). The data is based on a questionnaire circulated among 73 defence officials in France, Germany, the United Kingdom and Brussels-based institutions. The results are analyzed through social network