This paper examines the concept of constituent power and constitutional form in Jurgen Habermas' legal philosophy. It argues that a concept of constituent power needs to be embedded in a constitutional theory that can explain the difference between legitimate law and a mere wielding of power. Theories operating with assumptions of a pre-legal and unbound constituent power are either pre-modern or a-historical. While Habermas' theory can convincingly spell out general terms for a legitimate constitutionalisation and legitimate law-making, however, it appears to be at the same time too thin and too thick with regard to two recent transformations of the democratic nation-state: Firstly, it cannot grasp the shift from enabling 'freedom' to upholding 'security' as the central description of the function of the nation-state. This shift has severe implications for the discourse on human rights and their a priori status as constraints on the popular sovereign: the security paradigm seems to trump the notion of inalienable individual rights and replace them with the rule that the end justifies the means. Secondly, the idea of a necessary internal link between public and private autonomy in Habermas' system of rights appears to be unable to explain the emergence of supranational and transnational law outside of a national legal community. In a different reading, however, it can serve as a normative yardstick for existing regulatory structures, and as an orientation for the elaboration of new forms and institutions that may reduce the obvious democratic deficits of supranational and transnational regulation.