ENThe Sapieha Palace in the Antakalnis district of Vilnius is extraordinary both in its historical significance and artistic quality. However, so far we do not have sufficient information about the interiors of the palace, which were considerably damaged and partly destroyed during 19th century reconstructions. During the first stage of the conservation works of the Sapieha Palace in 2012, the previously inaccessible deep-lying parts of architectural elements and previously invisible areas of old masonry and plaster were gradually revealed, and the surviving fragments of sculptural decor and mural painting were exposed on the walls. It was established that the Sapieha Palace was built in the process of reconstruction of the former building, which, in turn, had experienced quite a few previous reconstructions. The first masonry palace on the present site of the Sapieha Palace may have been built in the late 16th century. The luxurious interior furnishing of the palace, which stood on the site of the present building from the late 16th century, is testified to by the surviving fragment of mural painting and luxurious tiles (part of them - with coats of arms), analogous to the ones found in other eminent historical buildings of Vilnius. According to available data, the reconstruction carried out by Jan Kazimierz Sapieha must have been the fifth in the architectural development of the old building. On both sides, corner turrets and galleries were built, and a new plane of the main (western) facade was formed after the western wall of the old south-western annex had been pulled down.While expanding his palace, Sapieha incorporated a large part of the northern facade, almost the entire eastern facade and the lower part of the southern facade without changing the size of windows and the rhythm of their arrangement. The fact that during the late i8th century reconstruction quite a large part of the former walls of the palace were retained partly determined the plan and spatial structure of the Sapieha Palace. The interior furnishing of the palace built by Sapieha in the late 17th century is at least partly reflected by the fragments of mural painting and stucco mouldings, which were uncovered during preliminary research, and which confirm the previous statements that the interior decoration works were carried out by the painter Michelangelo Palloni and the sculptor Pietro Perti. Important data for the reconstruction of the interior furnishings were provided by the fragments of tiles and decorative plates discovered in the conservation process, which allowed us to reconstruct some of the tile stoves mentioned in the sources, and provided more concrete information about the features of the Dutch faience plates which decorated the walls of two rooms. The existing data on the interiors were supplemented by the recently found inventory of the Sapieha Palace of 1790, which helped to specify the function of the rooms, the types and features of the ceilings, and the features of the flooring. The plans of the Sapieha Palace and the descriptions of its interior spaces corrected on the basis of the research carried out during the conservation works are presented in the appendix compiled by the architect Evaldas Purlys. [From the publication]