The border is one of the most urgent issues of our times. We tend to think of a border as a static line, but recent bordering techniques have broken away from the map, as governments have developed legal tools to limit the rights of migrants before and after they enter a country's territory. The consequent detachment of state power from any fixed geographical marker has created a new paradigm: the shifting border, an adjustable legal construct untethered in space. This transformation upsets our assumptions about waning sovereignty, while also revealing the limits of the populist push toward border-fortification. At the same time, it presents a tremendous opportunity to rethink states' responsibilities to migrants. This book proposes a new, functional approach to human mobility and access to membership in a world where borders, like people, have the capacity to move.