Saint Peter's Basilica is a church built in the Renaissance style located in Vatican City, the papal enclave that is within the city of Rome.

Designed principally by Donato Bramante, Michelangelo, Carlo Maderno and Gian Lorenzo Bernini, St. Peter's is the most renowned work of Renaissance architecture[2] and the largest church in the world by interior measure. While it is neither the mother church of the Catholic Church nor the cathedral of the Diocese of Rome (these equivalent titles being held by the Archbasilica of Saint John Lateran in Rome), St. Peter's is regarded as one of the holiest Catholic shrines. It has been described as "holding a unique position in the Christian world" and as "the greatest of all churches of Christendom."

Catholic tradition holds that the basilica is the burial site of Saint Peter, chief among Jesus's apostles and also the first Bishop of Rome (Pope). Saint Peter's tomb is supposedly directly below the high altar of the basilica. For this reason, many popes have been interred at St. Peter's since the Early Christian period. A church has stood on this site since the time of the Roman emperor Constantine the Great. Old St. Peter's Basilica dates from the 4th century AD. Construction of the present basilica began on 18 April 1506 and was completed on 18 November 1626.

Catholic tradition holds that Peter, after a ministry of thirty-four years, travelled to Rome and met his martyrdom there along with Paul on 13 October 64 AD during the reign of the Roman Emperor Nero. His execution was one of the many martyrdoms of Christians following the Great Fire of Rome. According to Origen, Peter was crucified head downwards, by his own request because he considered himself unworthy to die in the same manner as Jesus.

The crucifixion took place near an ancient Egyptian obelisk in the Circus of Nero. The obelisk now stands in St. Peter's Square and is revered as a "witness" to Peter's death. It is one of several ancient Obelisks of Rome.

According to tradition, Peter's remains were buried just outside the Circus, on the Mons Vaticanus across the Via Cornelia from the Circus, less than 150 metres (490 ft) from his place of death. The Via Cornelia was a road which ran east-to-west along the north wall of the Circus on land now covered by the southern portions of the Basilica and St. Peter's Square. A shrine was built on this site some years later. Almost three hundred years later, Old St. Peter's Basilica was constructed over this site.

The area now covered by the Vatican City had been a cemetery for some years before the Circus of Nero was built. It was a burial ground for the numerous executions in the Circus and contained many Christian burials because for many years after the burial of Saint Peter many Christians chose to be buried near Peter.

In 1939, in the reign of Pope Pius XII, 10 years of archaeological research began under the crypt of the basilica in an area inaccessible since the 9th century. The excavations revealed the remains of shrines of different periods at different levels, from Clement VIII (1594) to Callixtus II (1123) and Gregory I (590–604), built over an aedicula containing fragments of bones that were folded in a tissue with gold decorations, tinted with the precious murex purple. Although it could not be determined with certainty that the bones were those of Peter, the rare vestments suggested a burial of great importance. On 23 December 1950, in his pre-Christmas radio broadcast to the world, Pope Pius XII announced the discovery of Saint Peter's tomb.

There are over 100 tombs within St. Peter's Basilica, many located beneath the Basilica. These include 91 popes, Saint Ignatius of Antioch, Holy Roman Emperor Otto II, and the composer Giovanni Pierluigi da Palestrina. Exiled Catholic British royalty James Francis Edward Stuart and his two sons, Charles Edward Stuart and Henry Benedict Stuart, Cardinal Bishop of Frascati, are buried here, having been granted asylum by Pope Clement XI. Also buried here areMaria Clementina Sobieska, wife of James Francis Edward Stuart, Queen Christina of Sweden, who abdicated her throne to convert to Catholicism, and Countess Matilda of Tuscany, supporter of the Papacy during the Investiture Controversy. The most recent interment was Pope John Paul II, on April 8, 2005.