To cope with increasing air traffic congestion, the European Union launched the Single European Sky in the early 2000s, intending to reform the European airspace which, at that time, was still fragmented by national frontiers. However, this has not been a purely civilian task, as military air transport is likewise affected. In fact, NATO has been concerned by the challenges facing the European Air Traffic Management environment due to the growing commercial aviation since its early days. For this, the European Defence Agency has been tasked to coordinate military views, thus opening a window of opportunity for EU-NATO relations. Both organisations promoting close coordination between the users of civilian and military airspace have built direct contact with each other. While examining how NATO and the EU are shaping their informal cooperation in this sector, the article considers the airspace community of users as a configuration of relations where inevitable inter-and intrainstitutional power play is involved to reform the European airspace.ARTICLE HISTORY