The theories considered in this chapter have been developed within the discipline of political science, although they have also been influenced, especially in the theory of elitism, by political sociologists – that is to say, people brought up in the sociological tradition who have turned their attention to political institutions. Political science began to develop a separate identity within the social sciences towards the end of the nineteenth century. It was first concerned primarily with ‘political theory’, a combination of political philosophy and the study of constitutions and constitutional procedures. From the beginning of the twentieth century, when the firs first studies of voting behaviour were made, political science has become increasingly concerned with the study of political behaviour, how decisions are actually made and the influences brought to bear on decision-making, but ideological issues remain close to the surface.