In this article, I address the question: what kind of normative principles should regulate the governance processes through which migration across international borders is managed? I begin by contrasting two distinct categories of normative controversy relating to this question. The first is a familiar set of moral controversies about justice within border governance, concerning what I call the ethics of exclusion. The second is a more theoretically neglected set of normative controversies about how institutional capacity for well functioning border governance (...) can best be achieved, concerning what I call the constitution of control of international borders. I argue that progress can be made in resolving controversies of the latter kind by applying a new normative theory of political legitimacy, distinct from the moral theories of justice routinely applied to ethics of exclusion controversies. On the ‘collective agency’ model of political legitimacy that I propose here, principles of political legitimacy have the regulatory role of combating complex collective action problems that may otherwise impede an institution’s collectively valuable functions. Through applying this theory, I sketch some provisional prescriptions for the design of international border governance institutions that may follow from the demand for strengthening their political legitimacy. (shrink)