For centuries the monetary affairs of the Roman Republic had rested in the hands of the Senate. These elite liked to present themselves as steady and fiscally conservative, but as the 19th-century historian of Rome Wilhelm Ihne remarked:
The aerarium (state treasury) was supervised by members of the government rising in power and prestige, the Quaestors, Praetors, and eventually the Prefects. With the dawn of the Roman Empire, a major change took place, as the emperors assumed the reins of financial control. Augustus adopted a system that was, on the surface, fair to the Senate. Just as the world was divided in provinces designated as imperial or senatorial, so was the treasury. All tribute brought in from senatorially controlled provinces was given to the aerarium, while that of the imperial territories went to the treasury of the emperor, the fiscus.
Initially, this process of distribution seemed to work, although the legal technicality did not disguise the supremacy of the emperor or his often used right to transfer funds back and forth regularly from the aerarium to the fiscus. The fiscus actually took shape after the reign of Augustus and Tiberius. It began as a private fund (fiscus meaning purse or basket) but grew to include all imperial monies, not only the private estates but also all public lands and finances under the imperial eye.
The property of the rulers grew to such an extent that changes had to be made starting sometime in the 3rd century, most certainly under Septimius Severus. Henceforth the imperial treasury was divided. The fiscus was retained to handle actual government revenue, while a patrimonium was created to hold the private fortune, the inheritance of the royal house. There is a considerable question as to the exact nature of this evaluation, involving possibly a res privata so common in the Late Empire.
Just as the Senate had its own finance officers, so did the emperors. The head of the fiscus in the first years was the rationalis, originally a freedman due to Augustus' desire to place the office in the hands of a servant free of the class demands of the traditional society. In succeeding years the corruption and reputation of the freedman forced new and more reliable administrators. From the time of Hadrian (117-138), any rationalis hailed from the Equestrian Order (equites) and remained so through the chaos of the 3rd century and into the age of Diocletian.
With Diocletian came a series of massive reforms, and total control over the finances of the Empire fell to the now stronger central government. Tax reforms made possible a real budget in the modern sense for the first time. Previously it had issued the tax demands to the cities and allowed them to allocate the burden. From now on the imperial government driven by fiscal needs dictated the entire process down to the civic level. Under Constantine this aggrandizement continued with the emergence of an appointed minister of finance, the comes sacrarum largitionum (count of the sacred largesses). He maintained the general treasury and the intake of all revenue until Constantine divided the treasury into three giving the prefect, count and the manager of the res privata their own treasuries. The treasury of the prefect was called the 'arca.' His powers were directed toward control of the new sacrum aerarium, the result of the combination of the aerarium and the fiscus.
The comes sacrarum largitionum was a figure of tremendous influence. He was responsible for all money taxes, examined banks, mints and mines everywhere, watched over all forms of industry, and paid out the budgets of the many departments of the state, the upkeep of imperial palaces and other buildings, supplied the Courts with clothing and other items. To accomplish these many tasks, he was aided by a large central staff and field force. Just below the comes sacrarum were the rationales, comptrollers, positioned in each diocese. They supervised the collection of all tribute, taxes, or fees. They were everywhere and omnipotent until Constantine demoted them after his reorganization of the palatine level ministries' competencies in the years 325-326 by restricting their activity to supervision of the collection made by the governors under the general supervision of the vicars. The rationales lost the last of their provincial field force of procurators between 330-337.
Only the praetorian prefects who were more powerful. His office, as vice-regent to the emperors, took precedence over all other civilian officials and military officers. They were chief finance officers of the empire. They composed the global budget and set the tax rates across the board. After Constantine's reforms they were directly responsible for the supply of the army, the supply of food stuffs to the capitals, the imperial armament factories, weaving and dye mills, the maintenance of the state post. The magister officiorum who was a kind of Minister of the Interior and State Security and the comes rerum privatarum could counter the political the comes sacrarum largitionum. The magister officiorum (master of offices) made all the major decisions concerning intelligence matters was not a fiscal officer and could not interfere with the operation of the sacrae largitiones and the res privata. The comes sacrarum largitionum gradually lost power to the prefects as more and more in kind taxes of his department were converted to gold. By the 5th century their diocesan level staff were no longer of much importance, although they continued in their duties.
The imperial estates and holdings were huge. They res privata was directly under the management of the RP. The patromonium', or imperial inheritance were lands leased to indivivuals. Both were under the jurisdiction of the comes rerum privatarum. In the West the rents and tax income was shared with the sacrae largitionum but not in the East. In the East the palace administration took over gradually post-450 and the RP was finally dissolved by Justinian's successors.