Abstract: This thesis examines the evolution of the junior officer positions of the Roman army in the period 91BC – AD14, and the motivations, background, and status of their holders. Two introductory chapters consider the nature of the available evidence and the way in which the agendas and survival patterns of our sources have influenced modern perceptions. There follow three chapters of diachronic analysis, each analysing the number of officer positions available, the roles and functions of the junior officers, and the status of the junior officer positions in the periods 91 – 50BC, 49 – 31BC, and 30BC – AD14, and finally three thematic chapters examining professionalism and other motivations for service, the perception of service as a junior officer, and the role of the municipal elite within the junior officer corps. In addressing these issues, the thesis challenges the modern view that the junior officer corps suffered a dramatic decline in status at the start of the first century BC through unwillingness to serve on the part of the Roman social elite. Instead, emphasis is placed on the important changes which occurred within the junior officer corps during the period 49 – 31BC, when the increasing demands for both manpower and loyalty among the warring commanders had a significant impact on both the junior officer positions and the men who held them, and which also led to innovation in the organisation of auxiliary forces. The reforms of Augustus that followed, and the junior officer corps of the earliest years of the Principate are also discussed, in particular the notable military innovations of the Augustan period and the role of the Italian and provincial equestrian ordo. Furthermore, this thesis analyses the development of professionalism within the junior officer corps and the perceptions of service as a junior officer as expressed in literature as well as in epigraphic and iconographic commemoration. The thesis concludes with a series of appendices which list all attested junior officers of the period, as well as those considered junior officers by modern authorities, with discussion of those officers whose careers or dating might be considered controversial.