Medieval music is difficult. When performed, its harmonies are often pretonal, its rhythms obscure, and its language frequenly Latin and religious. Its theoretical framework is an abstract language of geometric proportions and arithmetic ratios. When Boethius articulated his taxonomy of music - a system that undergirded all Medieval musicological writing - he divided it into three types, and the first two made no sound at all: musica mundana was the music of the spheres, a harmony produced by the motion of celestial bodies so rarified that human ears did not have the ability to perceive it; musica humana was also phenomenally silent, referring to the continuous musical performance of the human soul as it resonated with the body. Only the third and final type, musica instrumentalis, was what we might consider "music" proper: sung or played melodies and harmonies. This dissertation argues for a fourth type of music implied, but almost never explicitly named, by Medieval thinkers, and one which generates much of Late Medieval English devotional and mystical literature. This is musica celestis, or, as I have chosen to call it, mystical song. Mystical song is produced by God and his angels in heaven - beings without corporeal bodies - and exists beyond the world of material being. However, beginning in the early fourteenth century, a number of English authors claimed to hear it during mystical experience. This dissertation charts the wide-ranging effects of musica celestis in devotional, mystical, liturgical, and literary writing from the beginning of the fourteenth century to the beginning of the Reformation in England.