Fresco is a technique of mural painting made on a wet plaster. Initially the larger part of St. Sophia Cathedral was decorated with frescoes (6000 sq meters). They were badly damaged during the retouchings of 18th and 19th centuries. After the foundation of the museum in 1934 the original frescoes were uncovered. Restorators managed to restore 3000 sq meters of frescoes. In some places fragments of later painting are left for saving the integrity of the pictorial ensemble.
St. Sophia Cathedral frescoes represent scriptural scenes, Gospel stories, individual images of saints (over 500). The walls and vaults of stair towers leading to the choir loft are decorated with a cycle of secular frescoes. Together with a group portrait of the Prince's family in the central nave of the cathedral they create unique example of old Ruthenian secular monumental painting in eastern Europe.