Interest in the great empires, including the early Roman Empire, never wanes. And yet, to this day, it is not fully understood how the Roman Empire was administered. Successive generations of researchers must address questions about how the administrative structures of such a vast empire were organised, how it was managed, who the imperial officials were, and what their duties and responsibilities were. This book is devoted to the office a rationibus, the main central institution responsible for imperial finances, and to the officials who staffed the office. The study not only presents one of the most important elements of the administration of the early Empire but also provides valuable insight into the functioning of that administration as a whole. Despite having been described by scholars since the 19th century, the officium a rationibus has never been the subject of a monograph unto itself. The publication almost every year of epigraphic sources and the need to reinterpret the well-known sources (mainly narrative) and opinions from historiography both warrant a new look at how this office functioned.