This research focuses on inter-organizational cooperation, a neglected theme of the international relations theory. The analysis of the European humanitarian politics emphasizes the factors, dynamics and effects of inter-organizational cooperation inside the ngos network voice and between this network and the European institutions. By linking different but complementary theories, the strategies of cooperation can be divided into three phases. If ngos gather to be recognized as reliable partners by intergovernmental organizations, the networks they create are characterized by a complex delegation structure which aims to meet the needs of their members and of the targeted institutions. The alleged flexibility of networks does not protect them from the pathologies of international organizations (path dependency, lack of adaptability).