Political science has traditionally reasoned in terms of the scientific study of political systems, where there is little place for assessment of the individual leader, but it will be contended in this chapter that the analysis of politics is incomplete without a correct identification and assessment of the contribution of individual political leaders. The study of political leadership assumes that leaders do make a difference: individual political leaders exploit opportunities created by particular sets of circumstances, make choices from a number of different available options, and exercise such unfathomable personal characteristics as political skill, courage and intelligence. Set against this, political leaderships in all advanced liberal democratic polities have to function within the context of powerful constraints, which limit their margins of manoeuvre. These include constraints relating to the domestic political system, such as the nature of the executive, the power of bureaucratic elites, the party and elective structures, the parliamentary configuration, the constitutional set-up, and corporatist factors. They also involve constraints relating to the economy (notably the interdependent relations between developed economies); the social structure (the limits to political activity imposed by the tolerance of society itself); and, not least, the international system and pattern of alliances. These constraints impinge heavily upon the freedom of manoeuvre of even the most powerful chief executives, although leaders do vary considerably in their capacity to deal with these constraints, and in the extent to which they admit their existence.