The exercise of effective control by one state in a territory of another state without the other state’s consent is subject to the law of occupation. In a global legal system that allocates public authority among sovereigns on a territorial basis and protects their respective entitlements, the law of occupation fills a “governance gap” that extends until the ousted government returns. The law must set limits to the exercise of public authority of the occupying state. Such an exercise of authority is inherently suspect due to the three-dimensional conflicts of interest that typically occur in occupations, between the occupier, the ousted government, and the occupied population. The literature has referred to the occupier as a “trustee,” most likely to underscore the occupier’s sensitive other-regarding duties in managing these conflicts. The law of occupation has evolved over time, reflecting changing perceptions concerning the source of sovereign authority (whether the prince or the people) and the constraints on the exercise of public authority (e.g., does the sovereign have unfettered discretion vis-à-vis its citizens or is it constrained by human rights obligations?). As a consequence, a study of the law of occupation offers a sort of “laboratory test” for examining the complexity – and the very viability – of a legal regime that expects one sovereign to act as a trustee of strangers. This chapter seeks to provide a general overview of the law of occupation.