Saint Michael the Archangel was one of three angels mentioned by name in the Scripture; a figure in early Christian, Jewish, and Islamic traditions; and is the patron saint of ambulance drivers. This enigmatic ecumenical figure was the subject of hagiographies, liturgical texts, and relics from Italy across Western Europe during the eighth century, culminating in a cult following. Despite this following, Michael has been poorly served in Anglophone religious and historical studies. This book explores the formation and diffusion of the cultus from c. 400-c.800, isolating its emergence within the orthodox traditions of the Greek-speaking East, and then following its development within Latin Catholicism. The cult of Michael, far from being extraneous or marginal, proves crucial to the formation of orthodox doctrine.