Hagia Sophia is the former Orthodox cathedral and mosque in Istanbul famous for its massive dome and Byzantine mosaics.
The present building was commissioned by the Byzantine emperor Justinian I after an earlier church was destroyed during the Nika riots. Designed by Anthemius of Tralles and Isidore of Miletus, it was completed in AD 537. Its central dome, supported by four pendentives, was a remarkable engineering achievement and helped define Byzantine architecture.
Hagia Sophia served as the cathedral of Constantinople for most of the Byzantine era, apart from its conversion into a Roman Catholic cathedral during the Fourth Crusade from 1204 to 1261. After the Ottoman conquest of Constantinople in 1453, it became a mosque. It was a museum from 1935 until its reclassification as a mosque in 2020.
Many Christian mosaics were covered after the Ottoman conversion, but important works were later uncovered or documented, including images of Christ, the Virgin Mary, and Byzantine rulers. Hagia Sophia is sometimes confused with Istanbul’s Blue Mosque or Süleymaniye Mosque, but those are Ottoman mosques rather than former Byzantine cathedrals.