This thesis argues that Ammianus is interested in, and attempts to define, a Roman identity applicable to his own multifarious world. It argues that Ammianus and some of his peers discern a clear increase in the number of foreigners and outsiders in the empire. While some of his peers lament this perception and adopt a hard-line approach, Ammianus has a much more nuanced view. It is argued that the model of Roman identity which he devises not only accounts for foreigners, but actually, in some cases, makes them exemplars of a flexible Roman identity based chiefly on the notion of appropriate behaviour. In this sense, his identity scheme is ultimately integrative and inclusive. As part of his definition of identity, Ammianus utilises an "outsider" perspective. This perspective is shown to dictate not only how he portrays his characters and their deeds, but even how he reflects on the substance of Romanness as a continual dialogue between "Roman" and "foreign", broadly conceived. It is finally argued that the historian's purpose in defining such an identity is to ensure that the eternity of the empire, in which he firmly believes, is safeguarded by future "Romans" who perhaps may never even see the City itself, but nevertheless remain committed to its protection.