This thesis is a revisionist account of the political history of the Roman Empire, from the later years of Constantine to the eve of Rome's disastrous invasion of Persia. It is a study of the reigns and governments of Constantius II, Constans, and Julian: of what they hoped to do and what they actually succeeded in doing. Necessarily, it is also the story of the men who staffed their administrations and led their armies, and of how emperors interacted with their servants. It is an attempt to probe the limits of what Roman rulers might achieve and understand the nature of the constraints under which they laboured: a study of political action and the context of political action. It is based on a comprehensive re-examination of the source material for the period: on the detailed work of re-dating, of re-categorising, and of critical analysis. At its simplest, its argument is very simple indeed: these three decades and their political history have been misunderstood. Constantius II and Constans were much more capable, much more interesting rulers than their caricatures in the sources, and cooperated successfully for their decade of diarchy. The terrible crisis which engulfed the Roman world after 350, reveals (paradoxically) not the instability and unpopularity of Flavian rule, but its success, the degree to which it had become the framework of empire. Julian was not merely a rather less starry figure than he would have us believe: he was an indifferent general and incompetent administrator. When, at last, supreme power was dropped into his lap, his reign saw the empire slide into chaos. This is the story of the fall of the House of Constantine, as it has not quite been told before.