The first part of this book sets out to answer three general question : Why should there be singing in Christian worship? How should music be coordinated with ritual action? What function does it fulfill therein? From these questions certain fundamental principles emerge: the act of singing is a mystery; singing is a ritual form of expression; music is the handmaid of the sacred text. The second part of the book will then try to explain the ritual function of music as deduced from the rite celebrated according to the mind of the Church. Here we shall meet a series of laws of function, laws which vary according to their intrinsically significant nature, according to the persons responsible for their performance, according to the literary texts which are to be sung and to the music which of its nature is suited to them. May these pages help both performers and composers of sacred music to a better understanding of, and love for, the world of music used by the Church to surround her worship for the greater glory of the true God.