Non-state actors play an important role in international law-making, law-adjudication and law-enforcement processes. However, little attention has been paid to the theoretical discourse about non-state actors and its relation to the doctrine of the subjects of international law. The articles collected together in this volume consider a range of issues on this subject, such as whether the solution lies in 'relativizing' the subjects or rather in 'subjectivizing' them, and contribute to the discussion to determine who may legitimately and authoritatively perform legally relevant acts on the international scene.