The Pantheon (Latin Pantheon, from Greek Πάνθειον Pantheon, meaning "Temple of all the gods") is a building in Rome which was originally built as a temple to all the gods of Ancient Rome, and rebuilt circa 125 AD during Hadrian's reign. The intended degree of inclusiveness of this dedication is debated. The generic term pantheon is now applied to a monument in which illustrious dead are buried. It is the best preserved of all Roman buildings, and perhaps the best preserved building of its age in the world. It has been in continuous use throughout its history. The design of the extant building is sometimes credited to the Trajan's architect Apollodorus of Damascus, but it is equally likely that the building and the design should be credited to the emperor Hadrian or his architects. Since the 7th century, the Pantheon has been used as a Christian church. The Pantheon is currently the oldest standing domed structure in Rome. The height to the oculus and the diameter of the interior circle are the same, 43.3 metres (142 ft). In the aftermath of the Battle of Actium (31 BC), Agrippa built and dedicated the original Pantheon during his third consulship (27 BC). Agrippa's Pantheon was destroyed along with other buildings in a huge fire in 80 AD. The current building dates from about 125 AD, during the reign of the Emperor Hadrian, as date-stamps on the bricks reveal. It was totally reconstructed with the text of the original inscription ("M·AGRIPPA·L·F·COS·TERTIVM·FECIT",standing for Marcus Agrippa, Lucii filius, consul tertium fecit meaning,

Dio's book appears to be the only near-contemporary writing on the Pantheon, and it is interesting that even by the year 200 there was uncertainty about the origin of the building and its purpose: Agrippa finished the construction of the building called the Pantheon. It has this name, perhaps because it received among the images which decorated it the statues of many gods, including Mars and Venus; but my own opinion of the name is that, because of its vaulted roof, it resembles the heavens. (Cassius Dio History of Rome 53.27.2) The building was repaired by Septimius Severus and Caracalla in 202 AD, for which there is another, smaller inscription. This inscription reads "pantheum vetustate corruptum cum omni cultu restituerunt" ('with every refinement they restored the Pantheon worn by age'). Since the Renaissance the Pantheon has been used as a tomb. Among those buried there are the painters Raphael and Annibale Carracci, the composer Arcangelo Corelli, and the architect Baldassare Peruzzi. In the 15th century, the Pantheon was adorned with paintings: the best-known is the Annunciation by Melozzo da Forlì. Architects, like Brunelleschi, who used the Pantheon as help when designing the Cathedral of Florence's dome, looked to the Pantheon as inspiration for their works.
Floor plan of the Pantheon from Georg Dehio/Gustav von Bezold: Kirchliche Baukunst des Abendlandes. Stuttgart: Verlag der Cotta'schen Buchhandlung 1887-1901.Pope Urban VIII (1623 to 1644) ordered the bronze ceiling of the Pantheon's portico melted down. Most of the bronze was used to make bombards for the fortification of Castel Sant'Angelo, with the remaining amount used by the Apostolic Camera for various other works. It is also said that the bronze was used by Bernini in creating his famous baldachin above the high altar of St. Peter's Basilica, but according to at least one expert, the Pope's accounts state that about 90% of the bronze was used for the cannon, and that the bronze for the baldachin came from Venice.[2]. This led the Roman satirical figure Pasquino to issue the famous proverb: Quod non fecerunt barbari, fecerunt Barberini ("What the barbarians did not do, the Barberinis [Urban VIII's family name] did"). In 1747, the broad frieze below the dome with its false windows was “restored,” but bore little resemblance to the original. In the early decades of the twentieth century, a piece of the original, as could be reconstructed from Renaissance drawings and paintings, was recreated in one of the panels.