This chapter explores how fiduciary principles have shaped international law from colonial times to the present. Fiduciary principles are evident not only in the text of the League of Nations Covenant and the United Nations Charter, but also in various subfields of international law, including the law governing U.N. missions, military occupation, the legal status and duties of states, and the role and responsibilities of diplomatic officers. In each of these contexts, the international community has affirmed that certain offices and institutions attract fiduciary duties under international law. Nonetheless, the international community has struggled to develop credible mechanisms for enforcing these fiduciary duties.