That’s a question with a complicated answer! Several architects ended up influencing the building of the basilica over the 120 years of building work. The design began with Donato Bramante, who won a competition to design the new basilica, and whose idea it was to base the shape on a giant greek cross, topped with a majestic dome similar to the Pantheon. Bramante was replaced in 1513 by a trio including Giuliano da Sangallo, Fra Giocondo, and Raphael, though the former two died in 1515, leaving Raphael to do the bulk of the work before his death in 1520. His successor, Baldassare Peruzzi, kept the internal structure of the apses, but he largely reverted to the greek cross plan, getting rid of the naves planned by Raphael. Antonio Sangallo the Younger then proposed a plan that combined elements of Peruzzi, Raphael, and Bramante’s ideas, and made sure to reinforce some of the internal architectural elements which had already begun to crack. Michaelangelo took over in 1547, unwillingly, at the insistence of Pope Paul, and is credited with bringing the design to the point where it could be successfully executed. The nave and facade were designed by Carlo Maderno, who also added the nave. Gian Lorenzo Bernini designed the Baldacchino (the pavilion which covers the central altar) and adapted Bramante’s original piers, turning them into niches with staircases leading to balconies. The balconies held the four most precious relics owned by the Vatican - the spear that pierced Christ’s side, the veil of Veronica, a fragment of the True Cross, and a relic of St. Andrew, brother of St. Peter. He also designed the monument that holds the chair of St. Peter, which was becoming to frail to be used.