This chapter discusses the total revolution in the nature of the imperial senatorial order. It considers 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. The most obvious institutional innovation of the fourth century was the creation of the senate of Constantinople. The new body did not spring fully formed from the head of the emperor Constantine, however, having at least three marked phases of development. The link between the bureaucracy and the senate was fully institutionalized in the reign of Valentinian I and Valens. The fundamental changes in the nature of the senatorial order naturally affected the type of careers being followed by its members. Individual senators and institutional bodies dominated by senators were involved in a wide variety of ways in imperial politics: the formulation of policy and regimes.