As one of four commonly accepted dimensions of the scope of application of international humanitarian law (IHL), the geographical scope defines where IHL is applicable. Based on a law of treaties analysis, the thesis examines the law of international armed conflict (IAC) first. It studies the prototypical IAC (i.e. the resort to armed force between states) as well as its variations (i.e. other situations to which the law of IAC applies). It argues that a nexus-based approach constitutes the most convincing approach for the determination of the law of IAC's geographical scope inside and outside the conflict parties' territories. The geographical scope of the law of non-international armed conflict (NIAC) is analysed second. As regards its applicability inside the territory of a state involved in or affected by armed conflict, additional possible approaches specific to the law of NIAC are examined and dismissed in favour of a nexus-based approach. The applicability of the law of NIAC to NIACs with an “extraterritorial element” is analysed last. It is in this context that contentious issues, such as the targeting of “fighters” on the territories of non-adjacent states, are discussed.