Since the end of the nineties, we attend a growing official interest on the international humanitarian questions. In spite of this recent fashion, the United Nations was always interested in the humanitarian field. As of its own creation in 1945 and even a little earlier with the practice of the UNRRA, the institutionalized international co-operation was a mean often used to deliver humanitarian assistance or another kind of assistance to vulnerable populations. The General Assembly and its subsidiary bodies, with the Economic and Social Council, are the bodies "constitutionally" - according to the Charter of the United Nations - responsible for the competence of co-operation in the humanitarian field. The more recent action of the Security Council in this field should be perceived only like one accidental fact, which must be looked carefully in order to clearly define its remit.The historical analysis of the practice of UNO is a useful way to highlight the content of a legal concept of interest, that of "international co-operation in humanitarian affairs". One of its most remarkable elements could be the system of the United Nations to provide and coordinate the humanitarian assistance to the victims of emergencies. This system is formed by several institutions and legal principles which must be respected by the Member States and the UNO itself. The vulgarizing of the humanitarian matters in the political speech of some States should not be confused with the "humanitarian function" that the same Member States have allotted to some UN institutions. UNO is not a humanitarian agency for the service of the great powers. UNO must keep its principal function of the maintenance of international peace and security. Except some precise cases, the humanitarian functions should remain in their traditional place: that of the humanitarian international co-operation.