44-14 BC Augustus establishes the Empire. 
64 AD Great Fire of Rome during the Nero's rule. 
68-96 AD Flavian Dynasty. Building of the Colosseum. 
3rd century Crisis of the Roman Empire. Building of the Baths of Caracalla and the Aurelian Walls. 
284-337 Diocletian and Constantine. Building of the first Christian basilicas. Battle of Milvian Bridge. Rome is replaced by Constantinople as the capital of the Empire. 
395 Definitive separation of Western and Eastern Roman Empire. 
410 The Goths of Alaric sack Rome. 
455 The Vandals of Gaiseric sack Rome. 
6th century Gothic Wars. 

By the end of the Republic, the city of Rome had achieved a grandeur befitting the capital of an empire dominating the whole of the Mediterranean. It was, at the time, the largest city of the world (and probably the largest city ever built until the nineteenth century). Estimates of its peak population range from 450,000 to over 3.5 million people with estimates of 1 to 2 million being most popular with historians. This grandeur increased under Augustus, who completed Caesar's projects and added many of his own, such as the Forum of Augustus and the Ara Pacis. He is said to have remarked that he found Rome a city of brick and left it a city of marble. Augustus' successors sought to emulate his success in part by adding their own contributions to the city. The Great Fire of Rome during the reign of Nero left much of the city destroyed, but in many ways it was used as an excuse for new development.

Rome was a subsidized city at the time, with roughly 15 to 25 percent of its grain supply being paid by the central government. Commerce and industry played a smaller role compared to that of other cities like Alexandria. This meant that Rome had to depend upon goods and production from other parts of the Empire to sustain such a large population. This was mostly paid by taxes that were levied by the Roman government. If it had not been subsidized, Rome would have been significantly smaller.

Rome's population declined after its peak in the 2nd century. At the end of that century, during the reign of Marcus Aurelius, a plague killed 2,000 people a day. Rome's population was only a fraction of its peak when the Aurelian Wall was completed in the year 273 (at that year its population was only around 500,000).

Starting in the early 3rd century, matters changed. The "Crisis of the third century" defines the disasters and political troubles for the Empire, which nearly collapsed. The new feeling of danger and the menace of barbarian invasions was clearly shown by the decision of Emperor Aurelian, who at year 273 finished encircling the capital itself with a massive wall which had a perimeter that measured close to 20km. Rome formally remained capital of the empire, but emperors spent less and less time there. At the end of 3rd century Diocletian's political reforms, Rome was deprived of its traditional role of administrative capital of the Empire. Later, western emperors ruled from Milan or Ravenna, or cities in Gaul. In 330, Constantine established a second capital at Constantinople. At this time, part of the Roman aristocratic class moved to this new centre, followed by many of the artists and craftsmen who were living in the city.

However, the Senate, while stripped of most of its political power, was still socially prestigious. The Empire's conversion to Christianity made the Bishop of Rome (later called the Pope) the senior religious figure in the Western Empire, as officially stated in 380 by the Edict of Thessalonica. In spite of its increasingly marginal role in the Empire, Rome retained its historic prestige, and this period saw the last wave of construction activity: Constantine's predecessor Maxentius built notable buildings such its spectacular basilica in the Forum, Constantine himself erected its famous arch to celebrate his victory over the former, and Diocletian built the greatest baths of all. Constantine was also the first patron of official Christian buildings in the city. He donated the Lateran Palace to the Pope, and built the first great basilica, the old St. Peter's Basilica.

Still Rome remained one of the strongholds of Paganism, led by the aristocrats and senators. When the Visigoths showed off before the walls in 408, the Senate and the prefect proposed pagan sacrifices, and it seems that even the pope was agreeable if this could help to save the city. However, the new walls did not stop the city being sacked first by Alaric on August 24, 410, by Geiseric in 455 and even by general Ricimer's unpaid Roman troops (largely composed of barbarians) on July 11, 472. The sackings of the city, which had remained untouched by barbarians since the times of Brennus, astonished all the Roman world. The fall of Rome was read as the definitive fall of the ancient order. Many inhabitants fled, and at the end of the century Rome's population may have been less than 50,000. In any case, the damage the sackings made has been probably overestimated. The city was already in a steep decline, and many monuments had been destroyed by the citizens themselves, who stripped stones from closed temples and other precious buildings, and even burned statues to make lime for their personal use. In addition, most of the increasing number of churches were built in this way. For example, the first St. Peter was erected using spoils from the abandoned Circus of Nero. This "self-eating" attitude was a constant feature of Rome until the Renaissance. From the 4th century imperial edicts against stripping of stones and especially marble were common, but the need of their repetition show how they were ineffective. Sometimes new churches were created by simply taking advantage of early Pagan temples, perhaps changing the Pagan god or hero to a corresponding Christian saint or martyr. In this way the Temple of Romulus and Remus became the basilica of the twin saints Cosmas and Damian. Later, the Pantheon, Temple of All Gods, become the church of All Martyrs.