This volume contains English translations of The Mystagogy of the Holy Spirit by Saint Photios the Great, his “Epitomes” of the Mystagogy summarizing its main points, and the Synodicon of the Holy Spirit, believed to have been composed in the thirteenth century, with introductions to both. It also includes the study “Saint Photios and the Filioque” by Father Michael Azkoul, tracing the historical development of the Filioque in the West; a detailed life of Saint Photios by Father Justin Popovich; and appendices providing the original Greek text of the Mystagogy and the Synodicon.