This work aims to analyze the meaning and functions of borders in the context of international migration. The article begins with a brief overview of the different concepts and historical evolution of borders, before focusing on their three core functions: legal-political, symbolic, and flow regulation. As a functional concept, borders cannot be understood without considering these three dimensions. A fourth function will then be added: to spatially circumscribe opportunities, put forward by cosmopolitan approaches to denounce the unjust consequences that such an arbitrary and morally irrelevant fact as the drawing of borders has on people’s life chances. Finally, the proposal of open borders is examined, and the need to rethink them from the perspective of global justice is asserted.