Maritime engagement in the Gulf of Aden is a puzzling case for anyone interested in the political and institutional problems underlying European Union–North Atlantic Treaty Organization (EU–NATO) cooperation. Although the EU’s operation NAVFOR ‘Atalanta’ and NATO’s ‘Ocean Shield’ operate in the same theatre and with similar mandates, there is no formal link between them. No joint planning has been envisaged, and no official task-sharing takes place. As this article aims to show, cooperation and coordination between EU and NATO forces at the operational and tactical levels have nevertheless worked surprisingly well. Two faces of EU–NATO cooperation become apparent: the political level is dominated by a permanent deadlock, while on the ground and at sea staff have developed a modus operandi that allows them to deliver fairly successfully in complementing yet detached operations. Based on 60 interviews with EU and NATO officials (2010–2013), this article illustrates how the operational and tactical levels have developed ways of coordinating efforts informally despite the lack of a formal framework. It aims to show to what extent and how they succeed at bypassing organizational boundaries and at overcoming political limitations. Although these practices are becoming increasingly institutionalized, it remains to be seen whether this will translate into formal changes.