It was built by bishop Ildebrando and with the Battistero of San Giovanni, or Baptistery, is a masterpiece of Florentine Romanesque architecture. It has a bell form and the façade is quite distinctive for the contrast between the white and the green marble it is surmounted by a splendid mosaic. The interior covers three levels, with the presbitery raised above the main level and partly buried crypt; there is a nave and two aisles, once richly frescoed.
The crypt was built in 1000 is made up of seven small aisles with cross-vault, supported by 36 small columns. From the presbitery you can reach the sacresty and the Chapel of Crucifix commisioned by Piero de’ Medici and built by Michelozzo and Luca della Robbia. The vault is decorated by tiles and with tondi of terracotta. The tomb of Cardinal of Portugal was sculputerd by Antonio Rossellino. On the right it is the palace of bishop, summer residence of Florentine bishops, until it was used as barraks by spanish troops of Cosimo I.