Drawing on material in the Dumbarton Oaks Byzantine Photograph and Fieldwork Archives, this catalogue records two periods of work that uncovered the mosaics of Hagia Sophia, hidden by whitewash and plaster for 400 years. The photographs and drawings record the procedures used in the restoration and the results of these projects carried out in 1848-49 by the architects Gaspare and Giuseppe Fossati and later in 1931-49 by the Byzantine Institute.