During the lifetime of Guillaume Du Fay (c. 1400-1474) the motet underwent a profound transformation. Because of the protean nature of the motet during this period, problems of definition have always stood in the way of a full understanding of this crucial shift. Through a comprehensive survey of the surviving repertory, Julie Cumming shows that the motet is best understood on the level of the subgenre. She employs new ideas about categories taken from cognitive psychology and evolutionary theory to illuminate the process by which the subgenres of the motet arose and evolved. One important finding is the nature and extent of the crucial role that English music played in the genre's transformation. Cumming provides a close reading of many little-known pieces; she also shows how Du Fay's motets were the product of sophisticated experimentation with generic boundaries.