Hagia Sophia (Holy Wisdom) was completed in 537 CE under the Byzantine emperor Justinian I, designed by the mathematician-architects Anthemius of Tralles and Isidore of Miletus. The building was the single largest interior space in the world for nearly a thousand years and remained the cathedral of the Patriarchate of Constantinople until the Ottoman conquest in 1453. It was then converted to a mosque; in 1934 the Republic of Turkey converted it to a museum; in 2020 it was reconverted to a mosque.

The dome, 31 meters in diameter at 55 meters above the floor, was engineered with a pendentive system that allowed the dome to rest on a square plan rather than a circular one. This solved a structural problem that earlier Roman architecture had not solved at this scale; the pendentive-supported dome became the canonical motif of Eastern Orthodox church architecture for the next millennium. The interior originally was clad with gold-ground mosaics, many of which survive under whitewash applied during the Ottoman period.

The building stands in Istanbul, on the site of two earlier churches destroyed by fire and riot. It is the foundational building of Byzantine architecture, the model for Ottoman imperial mosques (the Süleymaniye, the Blue Mosque) designed by Mimar Sinan in deliberate response, and one of the half-dozen buildings whose architectural influence runs continuously through 1500 years of practice across two religious traditions.