This essay examines the activities of the Institute of International Law (the Institute or IIL) during its 150 years of existence, dealing directly or indirectly with the colonial phenomenon. It distinguishes between two major periods of roughly equal length: first the period between the years 1873 and 1945; and second, the period from 1945 to the present. These correspond to two important periods of international relations: the second wave of colonial expansion and its remnants and the regime of mandates following World War I, on the one hand, and the era of the United Nations Charter, the promotion of human rights, self-determination, and decolonialization, on the other hand.