This article examines why the European Union (EU) is recognized as a relevant actor in some international institutions, but not in others. Drawing on theories of international institutions and corporate action, it develops a theoretical approach toward EU actorness that demonstrates under which conditions third parties gain an interest in recognizing this actor as a relevant party to international institutions and how the EU can become an actor in its own right. The EU is expected to be recognized as a relevant actor in an international institution if it has acquired action capability in the relevant governance area, while formal status plays an inferior role. This hypothesis is subsequently assessed for six international institutions that vary regarding the degree of EU action capability and the EU's formal status, including the WTO and IMF, FAO and WHO as well as two international environmental regimes. Empirical results confirm the fruitfulness of the theoretical approach.