Yet the role of international law is undermined by the fragmentation of legal norms scattered throughout a wide array of rules from numerous branches of international law (e.g., refugee law, human rights law, humanitarian law, labor law, trade law, maritime law, criminal law, nationality law). During the last two decades, a substantial number of textbooks have been published to map this growing field of international law.1 With this aim, international migration law may be defined as the set of international rules governing the movement of persons between states and the legal status of migrants within host states. Customary international law unveils and regulates the three components of the migration continuum: departure from the country of origin, admission into the territory of a foreign state, and sojourn therein.2 Regarding the first component, the customary law nature of the right to leave (with the usual lawful restrictions based on national security and public order) finds strong support in the large number of treaties restating this basic right, the similarity of their respective wording, their wide ratification, and the rare reservations, as well as in its endorsement in many international resolutions and domestic constitutions. ...of a long normative process, departure has been divorced from admission to constitute a distinct norm primarily addressed to states of origin and reinforced by the right to return to one's own country. The admission of non-citizens is governed by customary international law through both substantive requirements (such as the principle of nonrefoulement) and procedural ones (including the prohibition...