Aerarium (from Latin "aes", in its derived sense of "money") was the name (in full, "aerarium stabulum" - treasure-house) given in Ancient Rome to the public treasury, and in a secondary sense to the public finances.
Aerarium populi Romani
The treasury contained the monies and accounts of the state finances. It also held the standards of the legions; the public laws engraved on brass, the decrees of the Senate and other papers and registers of importance. These public treasures were deposited in the temple of Saturn at the Forum Romanum, on the eastern slope of the Capitoline Hill. During the republic, they were in the charge of the urban quaestors, under the supervision and control of the Senate.
This arrangement continued (except for the year 43 BC, when no quaestors were chosen) until 28 BC, when Augustus transferred the aerarium to two praefecti aerarii, chosen annually by the Senate from ex-praetors. In 23 these were replaced by two praetors (praetores aerarii or ad aerarium), selected by lot during their term of office. Claudius in 44 restored the quaestors, but had them nominated by the emperor for three years. In 56, Nero substituted two ex-praetors selected under the same conditions.
In addition to the common treasury, supported by the general taxes and charged with the ordinary expenditure, there was a special reserve fund, also in the temple of Saturn, the aerarium sanctum (or sanctius). This fund probably originally consisted of the spoils of war. Afterwards it was maintained chiefly by a 5% tax on the value of all manumitted slaves. This source of revenue was established by a lex Manlia in 357. This fund was not to be touched except in cases of extreme necessity.
Under the emperors, the Senate continued to have at least the nominal management of the aerarium, while the emperor had a separate exchequer, called fiscus. However, after a time, as the power of the emperors increased and their jurisdiction extended until the Senate existed only in form and name, this distinction virtually ceased.
Besides creating the fiscus, Augustus also established in AD 6 a military treasury (aerarium militare) as a fund for veterans' retirement benefits. It was largely endowed by the emperor himself and supported by the proceeds of new taxes, an inheritance tax and a sales tax on auctions. Its administration was in the hands of three praefecti aerarii militaris. At first these were appointed by lot, but afterwards by the emperor, from senators of praetorian rank, for three years.
The tribuni aerarii ("tribunes of the treasury") have been the subject of much discussion. They are supposed by some to be identical with the curatores tribuum, and to have been the officials who, under the Servian organization, levied the war-tax (tributum) in the tribes and the poll-tax on the aerarii. They also acted as paymasters of the equites and of the soldiers on service in each tribe. By the lex Aurelia (70 BC) the list of judices was composed, in addition to senators and equites, of tribuni aerarii. Whether these were the successors of the above, or a new order closely connected with the equites, or even the same as the latter, is uncertain.
According to Theodor Mommsen, they were persons who possessed the equestrian census, but no public horse. They were removed from the list of judices by Julius Caesar, but replaced by Augustus. According to Madvig, the original tribuni aerarii were not officials at all, but private individuals of considerable means, quite distinct from the curatores tribuum, who undertook certain financial work connected with their own tribes. Then, as in the case of the equites, the term was subsequently extended to include all those who possessed the property qualification that would have entitled them to serve as tribuni aerarii.
Prior to the decemvirate in 451 BC, there was a separate institution known as the publicum.
On a number of occasions it is recorded that various patricians incurred the anger of the plebs by paying the spoils from war into the publicum rather than the aerarium, for example Quintus Fabius Vibulanus in 485 BC following a victory over the Volsci and Aequi. From this it has been argued that the publicum was a fund administered by the patricians, but this has been disputed by others.