This is the first comprehensive study of Greek language ordinary chants ( Gloria/Doxa, Credo/Pisteuo, Sanctus/Hagios and Agnus Dei/Amnos tu theu ) in Western manuscripts from the 9th to 14th centuries. These chants – known as “Missa Graeca” – have been the subject of academic research for over a hundred years. So far, however, research has been almost exclusively from a Western point of view, without knowledge of the Byzantine sources. For the first time, this book presents an in-depth analysis of these chants and their historical, linguistic and theological-liturgical environment from a Byzantine perspective. The new approach enables the author to refute numerous (and largely contradictory) theories on the origin and development of the Missa Graeca and provides new answers to old questions.