The EU functions as a receptor and promoter of customary international law (CIL). However, from a theoretical (and international law) perspective, the questions why and when the EU is bound by CIL still have not been answered in definite terms. This chapter argues that the EU should be afforded a similar treatment as is afforded to (newly independent) states. This is based on a close examination of its practice in the field of international humanitarian law (IHL), where the primary organization tasked with ensuring peace and security – the United Nations – has so far avoided explicitly affirming that it is bound by customary IHL. After showing that it is indeed the EU which might bear direct responsibility in the context of military operations authorized under the Common Security and Defence Policy, the chapter examines the EU’s behaviour when it comes to new regulatory efforts, its executive and operational conduct, and the CJEU’s case law. On the basis thereof, it is concluded that the EU’s practice confirms the theoretical conclusions reached, and that therefore any theoretical uncertainties on whether and why international organizations are bound by CIL might in future be answered by paying closer attention to their practice.