In its current form, the Co-Cathedral of Cortona was designed by followers of Giuliano da Sangallo in the second half of the 15th century. Inside the church, a rich historical-artistic and devotional heritage is preserved, including the moving 15th-century terracotta Pietà, the canvas depicting the Nativity by the renowned Pietro da Cortona, and a mosaic by the Cortonese artist Gino Severini depicting the Sacred Heart. Paintings of remarkable value, also attributed to the workshop of Signorelli, adorn the walls of the choir. The Museo Diocesano of Cortona was established by the will of Bishop Giuseppe Franciolini and the Cathedral Chapter, gathering within it paintings on wood and canvas of extraordinary beauty, along with liturgical furnishings, reliquaries, and sacred vestments of remarkable historical and artistic value. Among them are the Annunciation by Beato Angelico, the Deposition by Signorelli, the Maestà and the large painted Cross by Pietro Lorenzetti, the Assumption by Bartolomeo della Gatta, the Vagnucci Reliquary, and the Passerini Vestments, created from designs by Raffaellino del Garbo and Andrea del Sarto.