In this review article, I suggest that the books under review should be conceived of as trailblazers of an emerging debate in international political theory. They all innovatively apply the concept of constituent power to the international realm and thereby contribute to establishing a new principle for the democratic legitimacy of international institutions. I argue that we should aim to develop a systematic theory of constituent power beyond the state because such a tool could considerably enhance the analytical categories of our research on democratic legitimacy beyond the state. Furthermore, I propose research questions for this new field of study, concerning conceptual, methodological and institutional aspects, and in turn apply these to the different conceptions of an international pouvoir constituant presented in the volumes under review in order to discuss their strengths and weaknesses.