This chapter looks at citizen assembly-style constitutional conventions as a path towards revitalising representative democratic institutions, with specific reference to the case of the United Kingdom. It explores three potentially controversial areas which require delineation before such a convention can be set up: ensuring its legitimacy, explored here from the point of view of its capacity to represent the relevant political community; entrusting it with a clear substantive mandate; and avoiding the alienation of political institutions. Fundamental constitutional change needs to be, or at least to be perceived as, legitimate if it is to take root. This is even truer in the UK considering the scale and scope of the reforms said to be needed in order to give coherence to the British constitution. One must remember that the high costs of popular constitution-making, including in terms of political will and citizen interest, will unavoidably render constitutional conventions exceptional.