The inlaid marble mosaic floor is one of the most ornate of its kind in Italy, covering the whole floor of the cathedral. This undertaking went on from the fourteenth to the sixteenth centuries, and about forty artists made their contribution. The floor consists of 56 panels in different sizes. Most have a rectangular shape, but the later ones in the transept are hexagons or rhombuses. They represent the sibyls, scenes from the Old Testament, allegories and virtues. Most are still in their original state. The earliest scenes were made by a graffito technique: drilling tiny holes and scratching lines in the marble and filling these with bitumen or mineral pitch. In a later stage black, white, green, red and blue marble intarsia were used. This technique of marble inlay also evolved during the years, finally resulting in a vigorous contrast of light and dark, giving it an almost modern, impressionistic composition.

The uncovered floor can only be seen for a period of six to ten weeks each year, generally including the month of September. The rest of the year, they are covered and only a few are on display.