While most contemporary armed conflicts are non-international, the application of international humanitarian law (IHL) to non-state armed groups (NSAGs), the mechanisms for holding them accountable for violations, and international engagement with them to promote humanitarian protection of civilians remain underdeveloped. A crucial question is how states and international actors can enhance engagement with NSAGs to improve respect for IHL, including through legal or quasi-‘legal’ tools and the socialization processes of norm diffusion, persuasion and social pressure. Section 2 of this article briefly charts the formal international laws and mechanisms that apply to NSAGs, noting the jurisprudential controversies over how and why IHL and human rights law does or should bind NSAGs. Sections 3 and 4 summarize the key reasons why NSAGs violate or respect humanitarian norms, based on existing research. Section 5 reviews the suite of legal tools and practical mechanisms available to encourage NSAGs to respect IHL. Section 6 then focuses on international efforts to engage NSAGs and the gaps in engagement. It concludes by offering tentative suggestions as to how the international community might deepen and widen its engagement with NSAGs to improve their respect for humanitarian norms.