It is a well-known fact that eunuchs became a heavy presence at the imperial Roman court in late antiquity, some attaining significant power in the service of the emperor. In this chapter I wish to reassert the evidence of Ammianus Marcellinus which is witness to the eunuch presence at court, but I will also suggest that the nature and depth of Ammianus’ interest in this subject has not been recognized sufficiently.1 Our historian discusses the origin of eunuchs, their nature, their character, their roles, their political influence as individuals and as a group, and uses them as a tool for political criticism, reflecting particularly on Constantius II and his court, but also on the decline of the Roman empire.