This article presents a theory of obligation in the context of humanitarianism. Its foundational assumption is that there exists a moral imperative to assist the structurally dispossessed and functionally abused. It builds particularly on the cross-disciplinary work (both academic and applied) of anthropologists, but also of political scientists, sociologists, human rights specialists, and others. The links between human rights and humanitarianism are stressed, while suggesting principles that can guide humanitarian organizations as they serve those in need. Humanitarianism is defined as “crossing a boundary;” risk usually is encountered by the service provider as scarce resources are used to help the vulnerable. Obligation is defined, in part, as “what one should do.” A theory emerges as the “morally possible” and the “materially possible” intersect. Notions of human dignity are shown not to be appropriate in orienting the real-world work of humanitarians; notions of fairness are more appropriate as humanitarian work is organized and implemented. “Pragmatic humanitarianism” occurs as principled guidelines and achievable actions merge, and as non-neutral stances are taken as (for example) refugees are assisted. Humanitarian aid is shown to be fundamentally a moral relationship based on the obligation of “those who have” to address the felt needs of “those who have not.” Examples from Bosnia are featured.