The gradual extension of the suffrage to all adult men and ultimately women too during the late nineteenth and early twentieth centuries transformed the politics of Western Europe and North America. Many contemporary theorists attributed these reforms not to any improvement in ordinary people’s political judgement because of better education and higher living standards, nor to a progressive appreciation of the right of all adults to be considered full citizens, but to a new social and economic reality having made such measures unavoidable. Quite simply, within a mass society political power could only be exercised with mass support. In spite of the inevitability of a widened franchise, many theorists believed deep tensions existed between the concepts of the ‘mass’ and ‘democracy’, rendering a ‘mass democracy’ almost a contradiction in terms. For the ideas of the ‘masses’ and ‘mass society’ were embedded within accounts of social organisation and behaviour that challenged the models of individual agency and rationality traditionally associated with democratic decision-making. Consequently, even democratically minded thinkers found that a coherent conception of mass democracy required a radical rethinking of the norms and forms of the democratic process (Femia 2001). This chapter traces the development of the new sociological and psychological languages of mass politics and their deployment in the construction of a modern theory of democracy. As we shall see, though still widely accepted, this theory incorporates empirical and normative assumptions arising from contentious and anachronistic views of human nature and society few would wish to espouse today.