It is the most populous municipality in Italy and the third largest in the European Union after Berlin and Madrid. With an area of 1,287.36 km², it is the largest municipality in the European Union and the fifth largest on the European continent, preceded by Istanbul, Moscow, London, and St. Petersburg. It is also the European municipality with the largest area of green space.

According to tradition, it was founded on April 21, 753 BC by Romulus (although recent excavations at Lapis Niger indicate the presence of settlements as early as two centuries earlier). Over the course of its three millennia of history, it was the first metropolis of the West, the beating heart of one of the most important ancient civilizations, which influenced the society, culture, language, literature, art, architecture, urban planning, civil engineering, philosophy, religion, law, and customs of the centuries that followed. It was the birthplace of the Latin language and the capital of the ancient Roman Empire, which extended its dominion over the entire Mediterranean basin and much of Europe. It was then the capital of the Papal States, subject to the temporal power of the popes, then of the Kingdom of Italy from 1871 to 1946, and finally of the Italian Republic.
 

It is known as Caput mundi, the Eternal City, and, par excellence, the Urbe.

The heart of Catholic Christianity, it is the only city in the world to host an entire state within its boundaries, the enclave of Vatican City, as well as the headquarters of the Sovereign Military Order of Malta, a legal entity under international law. Its historic center, bounded by the perimeter of the Aurelian Walls, a superimposition of evidence from almost three millennia, is an expression of the historical, artistic, and cultural heritage of Western Europe, so much so that, in 1980, together with the extraterritorial properties of the Holy See in the city, it was included in the UNESCO World Heritage List, a measure extended in 1990 to the territories within the Gianicolense Walls.
 

In 2007, the Colosseum, the city's most famous symbol and monument, was included among the new seven wonders of the world.

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