Sancta Sophia is the most interesting building on the world's surface. Like Karnak in Egypt, or the Athenian Parthenon, it is one of the four great pinnacles of architecture, but unlike them this is no ruin, nor does it belong to a past world of constructive ideas although it precedes by seven hundred years the fourth culmination of the building art in Chartres, Amiens, or Bourges, and thus must ever stand as the Supreme monument of the Christian cycle. The attempt here is some disentanglement of the history of the Church and an analysis of its design and construction; on the one hand, we have been led a step or two into the labyrinth of Constantinopolian topography, on the other, we have thought that the great Church offers the best point of view for the observation of the Byzantine theory of building.