This book traces the evolution and transformation of the genre of Latin-texted vocal music known as the motet during the lifetime of the leading composer of the age, Guillaume Du Fay (c. 1400-1474). The book includes an account of internal and external forces that influenced the transformation of the motet and of fifteenth-century music more generally. At the same time it provides a method for the interpretation of individual works based on recognition of the features and associations of the various subgenres of the motet.