The collection is made up of masterpieces from the Sienese school of the 14th and 15th centuries including paintings by Duccio di Buoninsegna, Simone Martini and the Lorenzetti brothers. The painters from Siena might not be as famous as the big stars from Florence (Michelangelo, da Vinci, etc.) but their painting is just as impressive, if less concerned with changing the world of art.
In the 1970s, the Spannocchi Collection was added which focuses on Northern and Flemish artists such as Durer. There is also now a Sala delle Sculture to house sculpture from the Sienese school of the 14th and 15th centuries.