Over the past decade, as threats to international peace and security have undergone important shifts, the legality of self-defence against attacks by nonstate actors directed from abroad has become a timely and controversial issue. At present, the debate is governed by two extreme positions. On the one hand, some argue that pursuant to article 51 UN Charter the right of self-defence can be exercised whenever an attack reaches a certain scale and effect. Others, however, state that the right of self-defence is restricted to situations where the attack can be attributed to a state. The present article attempts to draft a comprehensive overview of the debate, by means of an analysis of developments in state practice and opinio iuris, as well as an examination of relevant jurisprudence of the International Court of Justice. Following this analysis and taking account of the rules governing state responsibility, the authors argue that both extreme positions should be rejected. It is suggested that the ‘substantial involvement’ threshold, adopted by the International Court of Justice in 1986, still stands, although its interpretation has to be widened to cover the harbouring of armed bands and the provision of logistical support.