This is a history of the auxiliary branch of the Roman military. From the preface: "The following essay is an attempt to deal with an interesting branch of Roman military history which has not previously been made the subject of an independent treatise. In a study of this kind, which relies largely upon epigraphical evidence to which additions are constantly being made, it is equally necessary that the scattered material available should at intervals be collected and utilized, and that the unfortunate collector should realize that his conclusions will inevitably be revised in the future in the light of fresh evidence. I hope, accordingly, that I have made some use of all sources of information available without acquiring or expressing excessive confidence in the finality of my deductions. Students of the military system of the Roman Empire may complain that a certain number of complicated questions are too summarily disposed of in the following pages, but if discussion of the evidence in detail has been occasionally omitted with the idea of keeping the size of this book within reasonable limits, I hope that I have been careful to indicate where uncertainty lies."