The work surveys the non-citizen contingents of the Roman imperial military, tracing their institutional origins in the Augustan reforms and then examining their numbers, internal organization, and regional distribution. Drawing heavily on inscriptions and military diplomas, the author analyzes recruitment practices, deployment patterns, and operational roles in both campaign and frontier defense, and offers a detailed account of equipment and tactical uses. The study concludes by assessing the later stresses on the Augustan system and includes appendices collating key epigraphic material and evidentiary notes.