The important contribution of Heinrich Isaac (ca. 1455–1517) to the genre of the proper of the mass has long been recognised. His work in this genre, collected in the monumental posthumously published Choralis Constantinus, was considered a landmark even in the sixteenth century. Yet Isaac’s magnum opus was by no means isolated. The mass proper played a much greater and more significant musical and symbolic role in the landscape of later-medieval and Renaissance music-making than is currently acknowledged. The present collection of fifteen essays offers new insights into both Isaac's mass propers themselves, which are still shrouded by many enigmas, and their context within broader later-fifteenth and sixteenth-century mass proper traditions. The circumstances under which Isaac's mass propers were composed, performed, and transmitted are discussed afresh, as is the striking late-sixteenth-century reception that the Choralis experienced. Studies of previously unknown or little-examined mass proper collections from countries as widely seperated as Portugal and Poland, as well as of the transformation of the genre in Lutheran territories and in the hands of William Byrd, show that Isaac's enterprise, though the largest of its kind, was built on and embedded in a strong and ongoing tradition of proper settings and cycles.