The sanctus (the "thrice holy" of Isaiah 6.3) is found in almost all eucharistic prayer, ancient and modern, and comprises the prayer recited over the bread and wine at the communion service. The origin of the sanctus as a constituent element in the eucharistic prayer is one of the unsolved mysteries of Christian liturgy, and the author of this study makes a careful investigation into its background and the instances of its occurrence in early Christian literature.