This thesis deals with the role of the concept of virtus in Roman historiography of the late Republic and early Empire. I shall argue throughout the study that analysing and tracing this concept in the works of the historians of this period take us to the very heart of the question of their appraisal both of political change and Roman identity. Understanding this moral appraisal does not mean just a better comprehension of their concept of virtus, but a new approach to their concept of history as magistra vitae. In the first chapter, I shall introduce some characteristics of the nature of historical writing and the approaches of ancient and modern historians. I shall be challenging some currents views on the complexity of evaluating ancient history by rhetorical and moral standards. In chapter II, I shall consider the concept of virtus in terms of its etymology and usage; I will then attempt to show the particular connection between virtus and Romanness. I will also develop and explain the concepts of virilis-virtus and humana-virtus and place them in their philosophical context. Chapters III, IV, V and VI will form part of what I have called 'Virtus in History', and in these chapters I shall deal with four historians. The first section is dedicated to Sallust and his analysis of political decline in relation to virtus. I shall attempt to assess Sallust's influential creation of moral language in the writing of history. Then, I will consider the connection of virtus as a means to preserve libertas in Livy's work, especially considering the author's time. Chapter V is concerned with Velleius' history and his view that the principate has re-established virtus in Rome. I shall concentrate on Tiberius for my analysis of virtus and challenge some traditional approaches to this author and his prince. Finally, in chapter VI, I will examine Tacitus' perception of the nature of the political change that Rome has undergone. I will show how the transformations in politics have a deep influence on the very idea of Romanness and how the disturbance of this concept leads to a more profound and internal interpretation of it.