This volume is a sequel to the work I published, several years ago, under the title, Byzantine Constantinople: the Walls of the City, and adjoining Historical Sites. In that work the city was viewed, mainly, as the citadel of the Roman Empire in the East, and the bulwark of civilization for more than a thousand years. But the city of Constantine was not only a mighty fortress. It was, moreover, the centre of a great religious community, which elaborated dogmas, fostered forms of piety, and controlled an ecclesiastical administration that have left a profound impression upon the thought and life of mankind. New Rome was a Holy City. It was crowded with churches, hallowed, it was believed, by the remains of the apostles, prophets, saints, and martyrs of the Catholic Church; shrines at which men gathered to worship, from near and far, as before the gates of heaven. These sanctuaries were, furthermore, constructed and beautified after a fashion which marks a distinct and important period in the history of art, and have much to interest the artist and the architect. We have, consequently, reasons enough to justify our study of the churches of Byzantine Constantinople.