Construction on this vast cathedral began in 1390; and the work, as you can see, still isn't finished more than 600 years later. The wings of the transept are missing and the facade is only partially decorated, lacking most of the marble that was intended to adorn it. The main doorway was carved in 1425 by the great Sienese master Jacopo della Quercia. Above the center of the door is a Madonna and Child flanked by saints Ambrose and Petronius, the city's patrons. Michelangelo, Giulio Romano, and Andrea Palladio (among others), submitted designs for the facade, which were all eventually rejected.
The interior of the basilica is huge. The Bolognesi had planned an even bigger church—you can see the columns erected to support the larger version outside the east end—but had to tone down construction when the university seat was established next door in 1561. The Museo di San Petronio contains models showing how it was originally supposed to look. The most important art in the church is in the fourth chapel on the left: these frescoes by Giovanni di Modena date 1410–15.