State creation and direct international territorial administration are now beyond their zenith. At present we increasingly witness how, under the impact of various forms of external influence, important changes affect institutional and constitutional structures within the state, i.e. without creating new states or without affecting the state's territorial integrity. These changes are often paired with the introduction of a new constitution and of a transitional period ('interregnum') at the end of which a new constitution enters into force ('reconstitutionalization'). The thesis represents the first attempt to systematically analyze conflict-related domestic interim governance ('DIG') –public power exercised by interim governments, transitional councils, etc., in states said to be in transition ('transitional authorities')– from an international legal perspective. It introduces and develops the concept of ius in interregno, which refers to the body of legal norms applicable to conflict-related DIG. Despite the wide variety of sources and contexts, it is possible to carve out a rudimentary but coherent ius in interregno applicable to DIG. The analysis focuses on norms and practices of the interregnum itself, while making largely abstraction of the origin and outcome of 'transitions': the journey becomes the destination. Based on a close observation of several transitions, the thesis unveils which norms apply to DIG. These norms concern, inter alia, limitations ratione temporis and ratione materiae to DIG; and the 'inclusivity' of DIG en lien with the principle of internal self- determination. To the extent that a ius in interregno crystallizes as a body of norms, transitional authorities cannot act as they please: they are not legibus soluti. Finally, the thesis argues that third states that wish to interact with transitional authorities must also respect the core of a ius in interregno. States increasingly conduct constitutional geopolitics by trying to impact the constitutional structure of another state, even without forcible intervention, and by hand-picking their favored oppositional transitional authority. Revisiting the principles of self-determination and non-intervention, a ius in interregno also tackles the delicate issue of indirect (non-forcible) regime change.