Despite the institutionalization of the norm for need-based humanitarian assistance in international humanitarian law, there is significant variation in the practice of this norm at the level of implementation. This gap between institutionalization and implementation is examined in the context of the policies and practices of international NGOs, the chief operational agencies of the international humanitarian regime. INGOs have interpreted this norm in terms of human rights frameworks. This interpretation structures the nature of interaction and contestation at the domestic level, and is shaped by INGO identity. In Sri Lanka, the adoption of a rights-based approach to humanitarian assistance led indirectly to NGOs becoming participants in the conflict and privileging the LTTE in order to preserve access and implement their mandates. Implementation, at the domestic level, is a path-dependent negotiation, shaped by both actor capacity and identity.