This introduction provides an outline of the shadowland of legality and illegality in which armed groups inhabit. It argues that this shadowland is created by the tensions that (i) exist between the relationship between international and domestic law (ii) arise out of the limited role that armed groups have in law-making processes and (iii) are created when armed groups are rulers of territories and populations. It then goes on to introduce the various contributions, explaining the purpose of the volume and detailing its approach.