This study assesses the rules of international law relevant to the use of force against non-State actors. The rules of international law on the use of force are the lynchpin of the project of international law for a more secure and peaceful world. Yet, as important as they are, the rules of international law on the use of force are also highly contentious. With the shift in the nature of conflicts from inter-State wars to conflicts involving non-State actors, and with the growth in the threat of global terrorism, the focus of the law on the use of force has shifted to the use of force against non-State actors. To assess the permissibility of the use of force against non-State actors, this study will focus on two grounds that have been advanced as bases for the extraterritorial use of force against non-State actors: the right of a State to act in self-defence and intervention by invitation. While there are other grounds that have been advanced for the extraterritorial use of force in international law, it is only in respect of these two grounds that the role of non-State actors has a significant influence on the legality or not of the use of force.