Constitutionalism is in a state of flux. The concepts, structures and certainties on which the modern constitutional imagination is founded are under strain. From Poland to the United States, Latin America to Brexit Britain, there is evidence of political, social and institutional developments that challenge core tenets of constitutional government: of a loss of public faith in the traditional sources of constitutional authority; of a rise in authoritarian political leaders or movements; and of a rejection by these leaders of the countervailing authority of either specific institutions or constitutional norms. That these developments tend towards the centralisation, personalisation and unification of political power poses a particular challenge for the separation of powers. This paper examines possible reasons why separation of powers thinking is under strain at present; and assesses potential strategies for meeting the populist challenge.