This paper contends that the relationship between democracy and modern constitutionalism possesses an underappreciated complexity, which, unless addressed, prevents us from embracing the challenge to constitutionalism or the possibilities open to it in today’s globalising world. That complexity is revealed by treating democracy as an incomplete ideal, referring both to the empirical incompleteness of democracy as unable to supply its own terms of application - the internal dimension - and to the normative incompleteness of democracy as guide to good government - the external dimension. This double-edged incompleteness explains the contingent necessity of modern constitutionalism. Constitutionalism is a necessary response to democratic incompleteness - seeking both to realise democracy (the internal dimension) and to supplement and qualify democracy (the external dimension). Yet, if incomplete democracy requires the accompaniment of constitutionalism, such incompleteness also means that democratic considerations cannot specify definitively the content of constitutionalism. The content of constitutionalism as a means to completing democracy, therefore, remains contingent upon other normative and practical considerations. Democratic incompleteness thus remains both the justificatory foundation for contemporary constitutionalism and the main reason for its inherent fragility. The paper proceeds by examining the relationship between democracy and constitutionalism along various internal, external and mixed dimensions, observing that some of the ways in which constitutionalism treats democracy recur over time and circumstance. Yet how democratic incompleteness manifests itself, and how constitutionalism responds to incompleteness, also evolves and alters, revealing the relationship between constitutionalism and democracy as iterative. The paper then concentrates on the iteration emerging from the current globalising wave. The fact that states are no longer either the exclusive sites of democratic authority or the only constitutional entities and sources compounds democratic incompleteness and complicates how constitutionalism responds. Nevertheless, it is argued, the historical role of constitutionalism in political modernity as key to addressing the double incompleteness of democracy persists under globalisation, as does democracy’s inability to supply all vital terms of constitutionalism. This continuity reflects how the deep moral order of political modernity, in particular the emphasis on individualism, equality, collective agency and collective self-improvement, remains constant while its institutional architecture, including the forms of its commitment to democracy, evolves. Constitutionalism, itself both a basic orientation and a set of design principles for that architecture, remains a necessary support for and supplement to democracy. Yet postnational constitutionalism, even more than its state-centred predecessor, remains contingent upon non-democratic considerations, so reinforcing constitutionalism’s normative and sociological vulnerability. This conclusion challenges two dominant but opposing understandings of the postnational constitutionalism of the global age - both that which indicts global constitutionalism because of its weakened democratic credentials and that which assumes that these weakened democratic credentials pose no problem for postnational constitutionalism, which may instead thrive through a heightened emphasis on non-democratic values.