Over 450 motets survive from the fifteenth and sixteenth centuries which were composed with multiple simultaneously sounding texts. The size of this repertory has been underestimated and its importance under-acknowledged. Narratives of the genre overemphasize early fifteenth-century (and earlier) polytextuality due to its association with arcane rhythmic structuring techniques while stressing a new musical-textual ideal later in the century. This thesis is the first attempt to address the repertory of polytextual motets from the late fifteenth and early sixteenth centuries as a whole. It resituates polytextuality as a central aspect of the genre even after the supposed rise of musical humanism. It suggests a new partitioning of the repertory based on different relationships of texts and cantus firmi. It proposes that the function of cantus firmi shifts during this period toward acting in dialogue with the text(s) of the other voices, even though this dialogic aspect fades away by the mid-sixteenth century. It engages in case studies on small groups of motets, in which the notation, composition, and texts of motets are analyzed, especially concerning cantus firmi as elements of musical structure and as bearers of liturgical, biblical, devotional, and other associations. While scholars have undertaken numerous analyses of individual motets, less common are case studies which ask both why certain texts and cantus firmi were combined and how they were integrated into the musical structure. The appendix includes a catalogue of the repertoire of polytextual motets and chansons with Latin cantus firmi over this period, with indexes by cantus firmus and composer. Also included are transcriptions of seven polytextual compositions without published editions. My research demonstrates the importance of polytextuality within the genre, the sophistication of the compositions using it, and its ability to provide commentary on a number of theological, devotional, political, and aesthetic issues.