Article 51 of the UN Charter, in aﬃ rming the inherent right of self-defence of each UN Member State “against which an armed attack has occurred”, clearly indicates that the concept of armed attack plays a key role in delineating the right of self-defence. The concept in question was not, however, deﬁ ned in the UN Charter, and no universally acceptable deﬁ nition has yet emerged either in practice or in doctrine. One of the fundamental questions to be addressed in this context is who must engage in armed activity for it to qualify as an armed attack. This question is of particular relevance today because of the threat of international terrorism and the expansion of the concept of armed attack through the inclusion of an act of terrorism. The article discusses in some detail the emerging legal framework for attribution of actions undertaken by non-state actors to states.