Located in Vatican City, the papal enclave within the city of Rome, the Papal Basilica of St Peter, popularly known as St Peter’s Basilica, is regarded as one of the holiest Catholic shrines and used to be the largest Christian church in the world till 1989, when it was exceeded in size by the church in Yamoussoukro, Cote d'Ivoire.

Designed by Donato Bramante as a replacement for the old Constantinian church, which had been erected around 320 AD, construction of the present building began in 1506 by Pope Julius II and completed after more than a century, in 1615 under Paul V. Built in the Renaissance style, the design, construction and decoration of Saint Peter's involved the contributions by the greatest Masters of the day, which include Alberti, Raphael, Bramante, Michelangelo, and Bernini.

Covering an area of 2.3 hectares and built out of travertine stone, St Peter’s Basilica is 452 feet high, 730 feet in length and 500 feet in width, with an interior length of just over 693 feet. The dome of St Peter’s, designed largely by Michelangelo and built by his pupil Giacomo della Porta, rests on four pendentives and massive piers, each 60 feet thick. In fact, Michelangelo increased the size and strength of the load-bearing structure without destroying the central unity of the original design by Bramante. With a total height of 448.1feet (136.57 m) from the floor of the basilica to the top of the external cross, it is the tallest dome in the world that dominates the skyline of Rome. Its internal diameter of 136.05 feet (41.47 m), which has a greater diameter by approximately 30 feet (9.1 m) (30 than the Constantinople's Hagia Sophia Church in Istanbul, completed in 537.

The elliptical forecourt encircled by a Doric colonnade ends at the façade of St Peter’s Basilica. The façade, designed by Maderno, is 376.3 feet (114.69 m) wide and 149.4 feet (45.55 m) high and is built of travertine stone, with a central pediment rising in front of a tall attic, surmounted by thirteen statues of Christ flanked by eleven of the Apostles, (except Saint Peter) and John the Baptist. The façade is flanked by to bell towers, the oldest bell dates from 1288.

Of the five portals from the narthex, the entrance or lobby area at the west end of the nave, three contain notable doors to the interior. The northernmost door is known as the Holy Door, which, by tradition, is walled-up with bricks and opened by the Pope, only for holy years such as the Jubilee year. The present bronze door was designed by Vico Consorti in 1950.

The St Peter’s Basilica is cruciform in shape, with an elongated nave in the form of a Latin cross. The central space of the massive building is crowned with one of the largest domes in the world. The nave is framed by wide aisles giving access to a number of chapels, which include the Chapel of the Madonna of Colonna, the Chapel of the Pieta, the Chapel of the Presentation of the Virgin, the Gregorian Chapel, the Clementine Chapel, the Chapel of Saint Sebastian and several other altars. Apart from that, there is the chapel of Confession, beneath the high altar.

The entire interior of the Basilica is lavishly and aesthetically decorated with marble, reliefs, architectural sculpture and gilding. It is richly adorned with numerous priceless treasures, created in marble and bronze by the greatest Renaissance sculptures. Set in alcoves within the four piers supporting the dome are the larger-than-life statues associated with the basilica's primary holy relics, which include Saint Helena holding the True Cross, by Andrea Bolgi; Saint Andrew with St Andrew’s Cross, by Francois Duquesnoy, Saint Veronica, holding her veil with the image Jesus’ face, by Francesco Mochi and Saint Longinus, symbolically holding a spear that infamously pierced the side of Jesus, by Bernini. The first chapel of the north aisle contains Michelangelo’s Pieta. The tomb of Pope Alexander VII, located towards the end of the south aisle and occupying an awkward position, in a niche above a doorway into a small vestry, is the work of Bernini, which is regarded as one of the greatest tombs of the Baroque Age.

There are more than 100 tombs within St. Peter's Basilica, including a good number located in the Vatican grotto, underneath the Basilica. Apart from 91 popes, they contain Saint Ignatius of Antioch, the Holy Roman Emperor Otto II and Cardinal Bishop of Frascati. In 1939, workers renovating the grottoes beneath St. Peter's, discovered a large burial site and a wall painted red, directly beneath the main altar. In a niche connected to that wall, they found the bones of a man and in 1968, Pope Paul VI announced that those bones belonged to St. Peter.

St Peter's Basilica, the most prominent building in the Vatican City, is described by the American Philosopher Ralph Waldo Emerson as the ornament of the earth. As a work of architecture, it is regarded as the greatest building of its age and is enlisted in the UNESCO World Heritage Site in 1984.