International Relations scholarship suggests that meanings embedded in the idea of selfdetermination have shifted over time. This scholarship also recognises that throughout the twentieth century self-determination has played a key role in the demise of empire and the ensuing formation of new states. This was the case with the conclusion of World War I, with the wave of post-World War II decolonisation, and following the break-up of the Soviet Union and Yugoslavia. However, whereas scholarship depicts self-determination as central to the legitimacy of new states and international society, it does not tell us much about what happens after statehood is recognised. The assumption that self-determination ends where sovereignty starts, the author argues, obscures the ambiguous role that self-determination has played before, and crucially after statehood is recognised. Invoked to universalise the model of the nation-state and sovereign equality, self-determination has concurrently involved exclusions and hierarchies, both domestically and systemically. The present thesis is concerned with this fundamental and yet largely neglected part of the story of the expansion of international society. More precisely, the author argues that twentieth century understandings and usages of the idea of self-determination point to the existence of a recurrent tension. This is a tension between the egalitarian aspirations of self-determination on one hand, and practices of hierarchy associated with self-determination on the other. For each of the 20th century waves of expansion of international society, this tension has been evident at three different levels of world politics. First, it has been embodied in the disciplinary role of international society, when self-determination was redefined, during each wave of state formation, as the standard of legitimate membership and statehood. Second, the tension has manifested itself at the domestic level of the newly “self-determined” states as boundaries of national political communities were delineated. Third, and as an implication of all this, the tension is found in the ordering of states within international society.