This thesis explicates and critically considers the various roles played by moral psychology within the work of John Rawls throughout his career. In the second half of the 20th Century, Rawls’s development of a sophisticated theory of justice in the social contract tradition played a significant part in reviving the study of normative political philosophy in the western world. Rawls argued that any theory of justice must be closely integrated with our best contemporary understanding of human psychology. Moral psychology is hence widely recognised to play an important role in Rawls’s overall theory. But the precise role played has not been adequately examined. In this thesis, I identify six roles which moral psychology plays within the structure of Rawls’s theory. Moral psychology must defend the idea that the model for a just society which Rawls proposes is realisable and stable (role #1). Moral psychology is also employed to explain how persons now have acquired what sense of justice they have (role #2). By showing that Rawls’s just society can be realised and is stable, moral psychology is then subsequently used in the justification of Rawls’s theory of justice – first by showing that such a society is not futile (role #3), and second by showing that the society is comparatively more stable than leading rivals (role #4). The account of the psychological capacities of the moral person is used to place the limit on the scope of justice (role #5). And moral psychological facts are also likely to be, in some sense, constitutive of the nature of morality for Rawls (role #6). These roles are discussed throughout various chapters. What alterations occur to the overall place of moral psychology following Rawls’s later embrace of political liberalism is also discussed. The overall aim of the thesis is to produce an accurate exegesis on these matters, and in doing so indicate just how important moral psychology is within Rawls’s theory, but also to indicate, clearly and starkly, just how much more psychological and sociological investigation needs to be done in if the theory is to be substantiated, given Rawls’s own criteria.