During the middle and late second century, aristocratic Roman society underwent a transition: from the apparently orderly and settled imperial court of Marcus, through the reign of Commodus, punctuated by plots and purges, to the brief imperial tenure of Pertinax, riven by dynastic scheming. This thesis uses a prosopographical study of the career of Pertinax, who was born the son of a freedman and rose to become emperor in 193, to examine the shifting socio-political structures of this imperial society. The goal of the thesis is twofold: firstly, to gain a deeper understanding of the process and consequences of the transition of aristocratic society from 160-93; secondly, to emphasise the career of Pertinax as representative of these changes. Apart from prosopography, the methodology revolves predominantly on the analyses of literary sources - textual and epigraphic - created within an aristocratic Roman context, although where necessary these sources are supplemented with numismatic and archaeological evidence. Such a methodology allows us to examine the structural characteristics of imperial society and enables the study of its responses and developments throughout the numerous crises and upheavals, internal and external, of the middle and late second century. Prosopography provides the central evidentiary framework for the thesis. The career of Pertinax, which took place under the emperors Marcus Aurelius, Lucius Verus, and Commodus (as well as his own brief three month reign), provides fertile ground for the prosopographical study of an imperial society in transition. His career has typically been treated in a supplemental or fragmentary fashion, and numerous problems deserve systemic treatment on historical and historiographical grounds. These problems in turn provide the cases studies within the thesis for discussions of the changing nature of imperial society and its relationship to the institution and person of the emperor.