This article assesses the EU’s performance as a security actor in the context of NATO, both at the institutional level and when acting as a bloc of member states within NATO. Informed by a definition of ‘performance’ as the ability to achieve preset objectives (effectiveness) in an efficient, relevant and viable manner, we observe that the EU’s performance in the context of NATO remains limited. This could be seen as a reflection of underlying political divergences between the two organisations, hampering systematic and formalised intra-institutional cooperation as well as effective cooperation between the EU member states in NATO. More importantly, it has resulted in the emergence of more informal and ad hoc EU practices in the context of NATO as well as parallel EU and NATO practices.