The sanctus (the 'thrice holy'of Isaiah 6.3) is found in nearly all ancient and modern eucharistic prayer. Its origin as a constituent element in this prayer is one of the unsolved mysteries of Christian liturgy, and the author of this study puts forward a comprehensive investigation into its background and sources. In making his enquiry, Dr Spinks shows that earliest attestations of its use derive from Syria and Palestine. This evidence is then matched against current theories of the origin of the eucharistic prayer, which leads the author to state his theories about the likely origin of the sanctus, and the implications this has for theories of anaphoral evolution. The use of the sanctus from the seventh century through to the present is described, a survey which takes note of the innovatory uses of Cranmer and Luther, and which places the study of the sanctus in a wide theological context.