It was built in 27 BCE, by Augustus' right hand man Marcus Agrippa, and later remodeled by the Emperor Hadrian (AD 125-128). This ancient temple was built as a temple to all the gods of the Roman state but has served as a Christian church since the 7th century. It is the only building from the Graeco-Roman world which has remained substantially intact and in continuous use throughout to the present day, and as it is still a functioning church, silence is requested during your visit. The Pantheon is celebrated for its large dome. From inside you'll see traces of the former bronze ceiling, melted down during the reign of Pope Urban VIII to make weapons for the fortification of the Castel Sant' Angelo. The hole in the center of the ceiling, though, is an original feature designed for architectural reasons (the dome would collapse without it.) If it happens to be raining, you should definitely go to the Pantheon to see the rain pouring into the building through the hole in the ceiling. There are holes in the ground that drain the water. The spectacular doors are Romans, but not the original. The original bronze beams of the pronao were melted down by Pope Urban VIII and might have been used to create Bernini's Baldacchino, or canopy, in Saint Peter's. The dome is the largest masonry dome in the world, larger than that of Saint Peter. The building now appears to be built in a depression, but this was not the original appearance. The street level at the sides and rear has risen about since its construction due to the accumulation of debris from 2,000 years of settlement. This has necessitated the maintenance of the deep trench that keeps the building from being buried. The Pantheon was free for many years, but fees began being charged in 2023.

The Pantheon (UK: , US: ; Latin: Pantheum, from Ancient Greek Πάνθειον (Pantheion) '[temple] of all the gods') is an ancient temple in Rome, Italy, originally built under emperor Augustus (27 BC–AD 14) and reconstructed under Hadrian (117–138); since AD 609, it is a Catholic church called the Basilica of St. Mary and the Martyrs (Italian: Basilica di Santa Maria ad Martyres).

The current Pantheon was built on the site of an earlier temple, which had been commissioned by Marcus Vipsanius Agrippa during the reign of Augustus. After the original burnt down, the present building was completed by the emperor Hadrian and probably dedicated c. AD 126. Its date of construction is uncertain, because Hadrian chose to re-inscribe the new temple with Agrippa's original date inscription from the older temple.

The building is round in plan, except for the portico with large granite Corinthian columns (eight in the first rank and two groups of four behind) under a pediment. A rectangular vestibule links the porch to the rotunda, which is under a coffered concrete dome made from Roman concrete (also called opus caementicium), with a central opening (oculus) to the sky. Almost two thousand years after it was built, the Pantheon's dome is still the world's largest unreinforced concrete dome. The height to the oculus and the diameter of the interior circle are the same, 43 metres (142 ft).

It is one of the best-preserved of all Ancient Roman buildings, in large part because it has been in continuous use throughout its history. Since the 7th century, it has been a church dedicated to St. Mary and the Martyrs (Latin: Sancta Maria ad Martyres), known as "Santa Maria Rotonda". The square in front of the Pantheon is called Piazza della Rotonda. The Pantheon is a state property, managed by Italy's Ministry of Cultural Heritage and Activities and Tourism through the Polo Museale del Lazio.