This article identifies two countervailing sets of norms – one promoting humanitarian engagement with non-state armed groups (NSAGs) in armed conflict in order to protect populations in need, and the other prohibiting such engagement with listed 'terrorist' groups in order to protect security – and discusses how this conflict of norms might affect the capacity of humanitarian organizations to deliver life-saving assistance in areas under the control of one of these groups. Rooted in international humanitarian law (IHL), the first set of norms provides a basis for humanitarian engagement with NSAGs in non-international armed conflict for the purpose of assisting populations under their control and promoting compliance with the rules of IHL. The second set of rules attempts to curtail financial and other forms of material support, including technical training and co-ordination, to listed 'terrorist' organizations, some of which may qualify as NSAGs under IHL. The article highlights counter-terrorism regulations developed by the United States and the United Nations Security Council, though other states and multilateral bodies have similar regulations. The article concludes by sketching ways in which humanitarian organizations might respond to the identified tensions.