This essay reviews the work of the Institut de Droit International, for the occasion of the 150th anniversary of its founding in 1873, that addresses the acquisition or status of territory during the epochs of imperialism, sacred trust of civilization, and territorial administration by or under the United Nations. Section A of this essay introduces the work in a summary fashion. Section B briefly describes the context for each epoch. Section C highlights the challenges and controversies in each epoch surrounding the related topics studied by the Institute. Section D analyzes the three resultant resolutions. Section E briefly comments on the impact of the work, and Section F concludes by taking a look into the future. By and large, the work of the Institute relating to acquisition or status of territory stays very close to the leitmotifs or practices of the different epochs, although one can glean glimpses of inching forward on the part of the Institute, or more so of certain members. Notable examples are its assessment that notice of occupation is required to make it effective and may be done in the normal way of giving publicity to official acts or through the diplomatic channel, and its view on the international mandates system that, “The communities under mandate are subjects of international law. They have a patrimony distinct from that of the mandatory State; they possess a national status, and they may acquire rights or be held to their obligations.” The Institute’s work can be considered solid “codification” efforts. As such, the work, just like any other private codification efforts, may conceivably contribute to the strengthening of the relevant practice, for good or ill, or for good and ill, as we can see that such practice in the past, together with the codification that aspired to reflect it, can never catch up with the future.