The European Union has developed and strengthened its relations with various international organisations as part of its ambitious foreign and security policy agenda, including with NATO. EU-NATO relations have been studied in several ways, including through the lens of their meaning for both organisations’ performance in the field. The current chapter departs from a different angle by looking at the long-term effects of EU/NATO interactions, where the focus is on the everyday practices – and not on one-shot performances or fulfilment of pre-set goals – of both organisations, whether formal or informal. In particular, it studies how the political paralysis in the formal cooperation between the EU and NATO under the ‘Berlin Plus’ agreement has, over time, contributed to the strengthening of informal, ad hoc cooperation among diplomats, military personnel and the international staff of both organisations, in Brussels and in the field. These evolving EU-NATO informal practices seem to be detached from institutional or national belonging, with implications for the understanding of inter-organisational cooperation.