This paper addresses the question of humanitarian engagement with the non-state armed groups that increasingly populate the zones in which humanitarian action takes place. In particular, it seeks to understand why some combatants react positively and consistently to humanitarian demands to meet access preconditions, while others respond erratically, decline to respond or are hostile. The paper looks less at how to negotiate with such groups, and more at the various types of non-state armed group with which negotiations are likely to be conducted. The ultimate objective is to determine the parameters of responsible humanitarian engagement – i.e., to investigate the scope of successful engagement, one which maintains minimal operational preconditions, such as security for aid workers, and to explore the available modalities of engagement. The paper is accordingly aimed at senior and mid-level managers in operational decision-making positions: people who have to evaluate the potential effectiveness of the intended engagement, rather than field-based staff involved in face-to-face access negotiations.