European security is at a critical juncture and many have called for a more coherent and efficient response, involving both the EU and NATO. However, the primary tool for EU–NATO cooperation, “Berlin Plus”, has been stuck in a political quagmire since the mid-2000s, making a lot of scholars to conclude that this cooperation is obsolete and outdated. This article is challenging this view by analysing a range of informal but regular interaction patterns that have emerged. Using practice theory, it sheds new light on and explores how EU and NATO staff at all levels engage in informal practices on various sites in headquarters in Brussels and in field operations. A study of EU–NATO cooperation as practice focuses on the everyday, patterned production of security as well as what makes action possible, such as (tacit) practical knowledge and shared “background” knowledge (education, training, and experience). The article also discusses the extent to which shared repertoires of practice may evolve into loose communities of practice that cut across organisational and professional boundaries.