The building of the Roman Pantheon was begun in 27 BC, commissioned by Marcus Vipsanius Agrippa. At that time, it was a classical temple, with a gabled roof supported by a colonnade. This building was destroyed by fire in 80 AD, and between 118 and 128 AD, it was completely rebuilt by Hadrian, the Roman Emperor of that era. The Pantheon was originally built with adjoining water gardens and baths, however these are no longer in existence. Some minor repairs and alterations were made by various Roman Emperors of the third century AD, however the building as it can be seen today is, with the exception of paintings added in the fifteenth century, almost identical to the building it was in the second century.

The word pantheon is derived from the Greek, and literally means "temple of the Gods. The original Pantheon was a temple dedicated to the seven deities of the state religion of Ancient Rome. However, since the seventh century, the Pantheon has been a Christian Church. In 609 AD, the Emperor Phocas gifted the Pantheon to Pope Boniface IV, who consecrated the temple and dedicated it to St. Mary and to all Christian martyrs. It was then renamed as Santa Maria ad Martyres. The building is still in use as a church in the present day, and is used mostly for important masses.

During the Renaissance, the Pantheon came to be used as a tomb as well as a church. Italian painters Raphael and Annibale Caracchi are buried there, as well as two Italian kings, Vittorio Emanuele II, and Umberto I. Also during this period, the interior of the Pantheon was decorated with paintings, the most well-known of which is the Annunciazione by Melozzo da Forli.

The Roman Pantheon is now the best-preserved building in all of Rome, and the oldest "important" building in the world to still have an entirely intact roof. The design of the building is attributed to Apollodorus of Damascus, a second-century Greek engineer and architect. While the method of construction of the Pantheon has never been determined, there is no doubt that it is a truly remarkable achievement.

The Pantheon as it can be seen today is a circular building, constructed primarily of concrete, with brick accents. The great dome that rises from the roof is made of concrete, as are the Corinthian columns that form the front of the building and support the gabled roof. The engraved bronze double doors that serve as the entrance to the Pantheon are a massive seven meters high.

The dome measures 43 meters at its widest point, and rises to a height of 22 meters above its base (or 43 meters above the floor). At the top of the dome is an oculus—a circular opening about eight meters in diameter, which is the only light source for the interior of the building. Except in the lowest part of the dome, there appears to be no evidence of any structural support for it. That the dome has remained intact and stable for well over 1500 years is a testament to both its exceptional design and the high quality of building materials that were used in its construction.

The vast interior of the Pantheon is a sharp contrast to the exterior, which, it must be admitted, looks rather plain. The lower parts of the interior are adorned with paintings and colored marble, and the sheer size of the room and its dome is awe-inspiring, particularly when you remember that it was constructed more than 1500 years ago. A visitor stepping inside the Pantheon for the first time will immediately notice a sense of warmth and light, and a feeling of harmony that stems from the perfectly-proportioned dome—the diameter of the dome and the interior beneath it is exactly equal to the distance from the floor to the oculus.

The Pantheon is easy to find in Rome, lying between Piazza Navona and the Trevi fountain, and the surrounding streets are well-signposted. No advance booking is required, and entry to the Pantheon is free. Even if you prefer to stay clear of crowded tourist spots, the Pantheon is a must-see when you're in Rome. If you want to avoid the crowds, visit early in the morning or late in the afternoon.