At the start of the fourth century, the senatorial order was composed of the relatively few men – perhaps some six hundred or so – who were members of the senate of the city of Rome. Senatorial status was essentially inherited, although, since the beginnings of the empire, there had been a steady trickle of new elections to the order from the greatest provincial families across the empire, so that not all were of Roman, or indeed Italian, origin. In the fourth century, nevertheless, some still claimed more or less direct descent from old republican grandees such as the Gracchi and Scipios. All, whatever their origins, had the same formal status and the same attendant privileges. As a body, the senate had little real political power, and its true importance resided in the men of whom it was composed. Many belonged to the wealthiest stratum of landowners within the entire empire, and this wealth brought power, influence and ambition in its wake. It was the wealth of the individuals, indeed, and not the power of the institution which underlay the numerous senatorial candidates for imperial office in the political anarchy of the third century.

By A.D. 400, much of this had changed. A second senate had been created in Constantinople, and the senatorial order itself had grown enormously in size. Senatorial status remained hereditary, but, in the course of the fourth century, it also became much easier for outsiders to acquire it, particularly by pursuing a career in the imperial bureaucracy. The order itself had been subdivided further into three grades, with varying rights and privileges for each grade. This total revolution in the nature of the imperial senatorial order sets the agenda for this chapter, which will consider the institutional changes put in place in the course of the century, the new career patterns which resulted, and the evolving political role of senators, both in central, imperial politics and in the governing of localities.