Rome was already the capital of an empire during the Roman Republic. Becoming the capital of the empire did not essentially change the function of the Forum Magnum as the Romans called it (Roman Forum is a modern term). Most of its temples t, the senate house and the Comitia (the meeting area of the popular assemblies)had already been built. The majeor new development was the construction of basilicae (plural basilica, public buildings were public and private business were conducted and trials were held).

With the the establishment of rule by emperors the structures of the Forum magnum built during the Republic were retained and new additions were made over the centuries. This forum retained its function as a civic centre, the centre of public, administrative, judicial and political life. However, its function as political centre declined in the 3th century AD.

Julius Caesar had his own forum built next to it (this was still during the republican period) and in the early days of rule by emperors four imperial fora (plural of forum were built - see below.

The forum of the republican days had the Comiium (an area where the popular assemblies met to vote) the Rostra, a platform for speeches made with the bows (rostra) of ships captured in a naval battle, two other platforms for public speeches (one was also called rostra, the other was called Graecostasis) the aerarium (the treasury) which has in the temple of Saturn, the Tabularium (the state archives) which was built on the slope of the Capitoline Hill, the regia (the residence of the Pontifex Maximus, the chief priest of the Roman state religion) the Cruria Julia (the second rebuilding of the senate house ordered by Julius Caesar; the two previous senate houses were called Curia Hostilia and Curia Cornelia) the Senaculum (which was a place where the senators gathered before the entering the Curia for formal summons). The basilicae were the Basilica Porcia, Basilica Opimia, and Basilica Aemilia. Julius Caesar commissioned the Basilica Julia, which replaced the earlier basilica Sempronia on a grander scale. Temples were, besides the temple of Saturn, the altar of Vulcan (the Vulcanal) the Temple of Felicitas, the Temple of Janus, the Temple of Castor and Pollux, the Temple of Vesta with the Atrium Vestae (the sanctuary of the vestal Virgins) next to it and the Shrine of Venus Cloacina. There was also the Tabernae Novae (New Shops) a square with two facing rows of shops which housed the bankers and mopney chargers. These shops were rebuilt during the imperial period.

During the period of rule by emperors new temples were built: the Temple of the Divine Julius (49 AD; Julius Caesar was deified after his death) the Temple of the Divine Augustus (37 AD, Augustus was also deified) the Temple of Vespasian and Titus (79 BC) the temple of Antonius and Faustina (146 AD) and the temple of the Divine Romulus (built by Maxentius in honour of his son in 307 AD). The Portico Dii Consentes (Portico of the Harmonious Gods) was built in the 1st century AD. Triumphal arches were built: the Arch of Augustus (29 BC) the Arch of Tiberius (16 AD) and the Arch of Septimius Severus (203 AD). A new basilica, the Basilica of Mauritius was built in 308 AD by Maxentius. The Column of Phocea, built in 608 AD was the last addition to the forum.

The role of the Forum Magnum as a political centre declined in the 3rd century AD. The Comitum had already lost significance when the vote by the popular assemblies was scrapped by Cesar. Although the senate remained important, it was turned into an instrument for the power of the emperor. In the third century AD the governing elite started to appear at the forum less frequently. They addressed their audiences in palace basilica. A basilica which could hold large audiences became a feature their palaces. Their palaces became "the forum made private."

As already mentioned, Julius Caesar had his own forum built. It was small 160 X 75 meters (525 X 246 ft) and only had a temple, porticoes and a row of shops. This set the precedent for other emperors to build their own forum. These were the Forum of Augustus (inaugurated in 2 BC), the Forum of Nerva (it was started by Domitian in 85 AD, but was completed by his successor Nerva in 97 AD) and the Forum of Trajan (112 AD). The Forum of Augustus and that of Trajan relocated many administrative offices from the Forum magnum as there was insufficient space there.