The city of Aquileia lies on a narrow strip of land between the coastal mountains of north-eastern Italy and the northern shore of the Adriatic Sea, in what was at the time of its founding the Roman province of Italia. During the Illyrian wars a fort was needed to prevent the Illyrians' incursions and in 181 BCE the site of Aquileia was chosen as a suitable place. Its name, according to a plausible enough legend, comes from the fact that an eagle (Latin: aquila) was seen flying over as the fort's main street was being constructed.

As a frontier town at the mouth of a navigable river and having a protected harbour with an adequate port under construction, Aquileia's economy benefitted from both the presence of the Roman army, especially after Marcus Aurelius made it the chief border city of the area in 168 CE, and from the opportunity of trade with peoples to the east, south-east and north. It rapidly grew to a population of about 100,000, perhaps even as many as twice that number, and was one of the wealthiest cities in the empire. It held a position that was as strategic for commerce as it was for the military, and all overland trade routes between Italy and the Balkans passed through it. Emperor Augustus used the city as his headquarters in 34 BCE and in 14 CE Tiberius set out from there, building the Via Gemina into the territory of the barbarian tribes of the Danube and conquering anything that fell beside it, in the best Roman tradition of conquering by building a road into the target's country. In a way, it was like an ancient version of St. Louis.

Aquileia's greatest role in history is that of a principal city of the Christian church. Christian tradition has it that the diocese of Aquileia was founded by St. Mark and that its bishops included martyrs St. Hermagoras and St. Hilarius. By the fourth century CE Aquileia had become a major centre of Christianity and an important starting point for missionaries evangelising in the north and east. St. Valerian's synod against Arianism that was held there in 381 CE left the diocese inordinately influential and in charge of a huge part of north-eastern Italy and Illyria.

Attila rampaged through it in 452 CE on his way to Milano and Pavia. According to one tradition, the city of Venice (Venezia) was founded the same year by Aquileian refugees who fled to Torcello island, though it may have been inhabited thirty years earlier. Perhaps Venice's association with St. Mark is derived from this connection with Aquileia. Attila practically razed the city and 48 years later, the Lombards would do pretty much the same. The aftermath of Attila's destruction of Aquileia became the setting for Giuseppe Verdi's 1846 opera bearing the conqueror's name.

Aquileia would never regain its full glory. It remained the seat of an independent-minded section of the Christian church, whose leaders assumed the title of Patriarch. It became part of Charlemagne's empire in the ninth century, and regained some influence and wealth in the eleventh century which culminated in the building of a cathedral that was consecrated in 1031. Aquileia eventually lost its authority to other, up and coming dioceses before the city was destroyed in an earthquake in 1348. Its religious leaders kept on the Aquileian title but the see was, for all practical purposes, located in Udine and it was effectively divided between Venice and Udine before Venice was formally awarded equal status. The Patriarchate of Aquileia finally met its end in name as well as substance in 1751.

Even before its official demise, the archaeological significance of Aquileia was recognised. Giandomenico Bertoli from Udine first collected items from Aquileia and exhibited them in his house. He also compiled a book describing them. The first proper museum was set up in 1807 and Bertoli's findings constitute the core of all museums' exhibits since. The church buildings built in the footprint of the 4th century basilica which formed the heart of the early Christian city was used as a Benedictine monastery for about 900 years, before being turned over to more mundane uses like wine-making in the 18th century. It was discovered to hold historical treasures in 1895 and eventually found its current use as a museum in 1961, and hosts pre-millennial ecclesiastical artefacts next to its impressive mosaic floor.

The city itself remains mostly unexcavated but is believed to be the single most complete ancient Roman city in existence. Most of what is dug up is covered again to reduce its exposure to the elements but a large amount of it has yet to be uncovered. Sites that are on display include the characteristically Roman necropolis and forum, port facilities, and several private and public buildings with quite well-preserved Roman artwork. Aquileia was designated an UNESCO World Heritage Site in 1998.

The modern town of Aquileia was known as Aglar in mediaeval times and is located near the port city of Trieste in the region of Friuli-Venezia Giulia. Administratively it belongs to Udine province and has about 3400 inhabitants.