When it was initially built, the Pantheon was meant to serve as an assembly hall where the public could gather. Hadrian would have sat in a throne to oversee public gatherings. While meant to represent all gods as its name suggests, the Pantheon was not necessarily used as a temple because it lacked the rectangular shape of previous temples.

In part, the Pantheon was meant to appease the gods because the destruction of the previous buildings in its place was seen as a sign that the gods were unhappy. In the niches would have originally stood statues of various gods, and deified emperors. For example, a statue of Augustus was placed in one of the main recesses.

The building is cylindrical with a portico of 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, with a central opening (oculus) to the sky. The Pantheon is one of the best-preserved of all Ancient Roman buildings, in large part because it has been in continuous use throughout its history.

circa 27 BCE-

Republican Period (27–25 BCE)
The original Pantheon was commissioned by Marcus Vipsanius Agrippa, a close ally of Augustus, around 27–25 BCE as part of a broader reconstruction of Rome following the Battle of Actium. Agrippa intended it as a temple dedicated to all Roman gods, though its precise function remains debated. The structure, noted for its rectangular shape and portico, was destroyed by fire in 80 CE, leaving only its inscription as a record of its construction.

Hadrianic Reconstruction (118–125 CE)
The current Pantheon was rebuilt under Emperor Hadrian between 118 and 125 CE, following another fire in 110 CE. Departing from Agrippa's design, Hadrian introduced a revolutionary architectural plan featuring a massive dome with an oculus. The inscription honoring Agrippa was retained as a nod to the temple's origins. The new Pantheon, blending traditional portico and innovative rotunda elements, symbolized the emperor's vision of unity between Rome's past and its imperial future.

Late Antiquity and Christianization (4th–7th Century CE)
By the 4th century, the Pantheon had lost its original function as a pagan temple. In 609 CE, Emperor Phocas donated the building to Pope Boniface IV, who consecrated it as the Church of St. Mary and the Martyrs, or Santa Maria ad Martyres. This Christianization ensured the preservation of the Pantheon during a time when many pagan structures were dismantled.

Medieval and Renaissance Revival (8th–16th Century CE)
Throughout the Middle Ages, the Pantheon remained a functioning church, protecting it from neglect. In the Renaissance, it gained renewed attention as a symbol of classical architectural brilliance. Artists such as Michelangelo admired its proportions, and its use as a burial site for prominent figures, including Raphael, elevated its cultural significance.

Modern Period (17th Century–Present)
During the Baroque period, the Pantheon saw additions like the bell towers commissioned by Pope Urban VIII, which were later removed due to public disdain. In the 19th century, it became a national monument and the burial site for Italian kings Victor Emmanuel II and Umberto I. Today, it stands as one of the best-preserved monuments of ancient Rome, a testament to the engineering genius of Roman architects and a continuous place of worship and inspiration.

circa 120 CE

Facade
Today the facade is fronted by eight monolithic columns topped by a frieze topped by a Greek-style pediment. The pediment was decorated with relief sculpture, probably of gilded bronze. The columns are made of red and gray granite and the shafts stand 40 Roman feet tall, carved in eastern Egypt. The building was originally approached by a flight of steps. Later construction raised the level of the ground leading to the portico, eliminating these steps.

The pediment crowning the portico was decorated with relief sculptures, likely crafted from gilded bronze. Archaeological evidence, including the holes marking the placement of clamps, suggests a central motif featuring an eagle within a wreath, with ribbons extending to the corners of the pediment. This decorative scheme symbolized imperial authority and divine favor.

circa 120 CE

Dome Exterior
The dome itself is created by overlapping barrel vaults over the third-story chambers. Almost two thousand years after it was built, the Pantheon's dome is still the world's largest unreinforced concrete dome. The dome spans 150 Roman feet; the oculus is 30 Roman feet in diameter; the doorway is 40 Roman feet high. The interior of the dome was possibly intended to symbolize the arched vault of the heavens. The dome features sunken panels (coffers), in five rings of 28. The oculus at the dome's apex and the entry door are the only natural sources of light in the interior.

circa 120 CE

Drum
The Pantheon’s drum, is a massive, cylindrical structure that forms the bulk of the monument. Though made of concrete, it is deceivingly light — the thicker sections of the wall accommodate empty, semicircular spaces. The drum’s solid appearance conceals its hollow interiors while maintaining its strength. The Romans invented and utilized a system of interlocking brick arches, vaults, and piers to enable the drum’s even weight distribution and support. From the outside, the monument seems dense but allows a hollow interior: this Roman technique can be seen in Trajan’s Column as well.

circa 120 CE

Portico or the Pronaos
The portico of the Pantheon was originally approached by a flight of steps, which were later removed when the ground level was raised during subsequent construction phases. This change significantly altered the visual approach to the structure, integrating it more seamlessly with its urban environment.

Analysis of the intermediate block between the portico and the rotunda reveals the remains of a second, higher pediment. This discovery suggests that the current portico is shorter than initially intended. The original design would have accommodated columns with shafts measuring 50 Roman feet (14.8 meters) tall and capitals 10 Roman feet (3 meters) in height. However, the present structure features columns with shafts 40 Roman feet (11.9 meters) tall and capitals 8 Roman feet (2.4 meters) tall.

Scholars, including Mark Wilson Jones, attribute this discrepancy to logistical challenges in delivering the taller columns. Builders adapted the design, making structural adjustments to fit the shorter columns. Rabun Taylor further posits that even if the taller columns had been delivered, spatial constraints during construction would have prevented their use. The process of pivoting columns upright required significant ground space on either side of the pediment, which would have been infeasible for the longer shafts.

The grey granite columns used in the portico were quarried in Egypt at Mons Claudianus and transported over vast distances. Each column, measuring 11.9 meters in height and weighing 60 tonnes, was dragged to the Nile River on wooden sledges, floated downriver during high water, and shipped across the Mediterranean to Ostia. From there, they were hauled up the Tiber River and moved to the Pantheon’s site using rollers or sledges.

The rear walls of the portico include two large niches, possibly designed to house statues of Augustus Caesar and Agrippa, underscoring the portico’s symbolic and political significance within the broader context of the Roman imperial identity.

circa 120 CE

Grottoni
The so-called grottoni is a somewhat rectangular structure built against the south side of the Pantheon's drum. This structure contains a number of structural reinforcements and buttresses that were built to support the weight of the rotunda, when a large crack appeared during construction during the reign of emperor Hadrian. The grottoni structure abuts the space between the north wall of the Basilica of Naptune and the south side of the Pantheon. Today this space houses some of the architectural artefacts from various paeriod of ancient Rome, and is usually not accessible to the public. The basic purpose of this structure was architectural rather than spatial.

Despite meticulous architectural planning and robust construction, the stability of the Pantheon was a significant concern for its builders. This is evident in the addition of ancillary structures known as the grottoni, positioned between the rotunda and the adjacent basilica to the south. These parallel walls, floors, and vaulting, spread over two levels, effectively formed a massive buttress system. A solid brick arch above the grottoni connected the basilica to the rotunda, further reinforcing the structure. The crude manner in which these elements abut the rotunda, without ceremonial or utilitarian purpose, underscores their role as structural countermeasures. Notably, the lower portions of the grottoni lack bonding with the rotunda, highlighting their later addition.

Evidence of a lost connection between the grottoni and the basilica is visible in the scarring on the rotunda’s exterior, particularly around the three openings at mid-height. While it has been traditionally assumed that the grottoni were constructed after the rotunda was completed, as a reactive solution to outward thrust, newer observations suggest a different timeline. The cornice of the “bridge” aligns with the rotunda’s middle cornice in a premeditated manner, implying simultaneous construction at higher levels. Arch springings, integral to the rotunda, further corroborate this coordinated building effort.

The rapid construction of the grottoni suggests an urgent response to early structural challenges. Comparisons with the swift pace of construction at Trajan’s Baths—where walls of comparable dimensions rose 15 meters in just two and a half months—indicate that the grottoni might have been built within a few years. Their early initiation aligns with the rotunda reaching approximately one-third of its final height, well before the dome’s inward curvature became significant.

The construction of the grottoni likely addressed a critical issue: a massive crack along the rotunda’s axis, near the apse where the wall was at its thinnest. This crack, unlike others that dissipate higher up, extended to the floor, possibly caused by foundation settlement. While a geotechnical investigation would be required to confirm this hypothesis, the grottoni appear to have been designed to mitigate further movement and counteract the dome’s thrust.

Ultimately, the grottoni served their purpose. Though their construction reveals the builders’ alarm at the Pantheon’s potential instability, their intervention ensured the enduring survival of this architectural marvel.

circa 120 CE

Rotunda
The height and diameter of the interior rotunda both measure 43.3 meters; the implication of this is that a perfect sphere with a same diameter would fit just perfectly inside the rotunda. The oculus, or opening at the top of the dome, measures 8.8 meters across and significantly lightens the load on the foundation of the structure. Serving as the major source of light in the Pantheon, the oculus also allows in rain and snow, setting a different atmosphere throughout the seasons. The floor is sloped towards drains that are present to collect rain. Blind windows line the rotunda, probably meant to let light into the extensive network of passageways that are used by maintenance crews. The marble work on the floor containing patterns of circles and squares is a 19th century accurate reproduction of the original floor.

circa 30 BCE

Basilica of Neptune
The Basilica dedicated to goddess Neptune (Basilica di Nettuno) was a basilica built in Rome by Marcus Vipsanius Agrippa in honour of Neptune and in celebration of his naval victories at Mylae, Naulochus and Actium. Near the site of the Pantheon, its remains were restored under Hadrian for an unknown use. It was part of building works on the Campus Martius between 33 and 25 BCE, possibly financed by the proceeds of Octavian's campaign in Illyria between 35 and 33 BCE. The project also included the Pantheon, the Saepta Iulia and the Baths of Agrippa.

circa 30 BCE

Portico of the Argonauts
The Porticus Argonautarum (Portico degli Argonauti) was an ancient structure in Rome. The building was located in the Saepta Julia, a large square in the Campus Martius used for public comitia (assemblies) and lined the western side of the Saepta Julia. The portico of the Argonauts was added in 25 BCE, to commemorate Agrippa's naval victories in 31 BCE: it took its name from its decorations, which depicted the mythological expedition of Jason.

circa 30 BCE

Macuteo Obelisk
This obelisk was originally erected by Rameses II as one of a pair at the Temple of Ra in Heliopolis, the other being the now much shorter Mattei Obelisk. The Macuteo Obelisk is 20.8 feet tall (47.6 feet with base), covered in hieroglyphics, and now stands in front of the Pantheon. The inscriptions contain phrases that indicate the kinship relation between the pharaoh and the Sun god ("Excellent son of the Sun god") and commemorate the works carried out in honor of the god. In 1711 it was erected in its present location on a pedestal decorated with a fountain with dolphins, adapting it to the 16th century fountain with masks by Giacomo della Porta.