St Peter's Basilica was built between 1506 and 1590, when the dome was finally completed. It is on the site of the tomb of St. Peter; his relics were finally found and authenticated in the middle of the 20th century. Before the current grand basilica, a 4th-century church built by Emperor Constantine stood here.
This is a church like no other. It is huge and full of significant artworks and tombs, including that of Pope John Paul II. One of the most beautiful pieces is the marble Pieta by Michelangelo just inside the door on the right. It is now behind bullet proof glass after being attacked by an art-hating lunatic in 1972.
If you can time your visit with a Mass, you will see the most important hierarchy of the Catholic Church come to worship in their red robes and hats. Climbing to the top of the dome gives a wonderful view over the piazza and Bernini's enclosing colonnade below, and across Rome.