The 'cyclic' polyphonic Mass has long been seen as the pre-eminent musical genre of the late Middle Ages, spawning some of the most impressive and engrossing musical edifices of the period. Modern study of these compositions has greatly enhanced our appreciation of their construction and aesthetic appeal. Yet close consideration of their meaning - cultural, social, spiritual, personal - for their composers and original users has begun only much more recently. This book considers the genre both as an expression of the needs of the society in which it arose and as a fulfilment of aesthetic priorities that arose in the wake of the Enlightenment. From this dual perspective, it aims to enhance both our appreciation of the genre for today's world, and our awareness of what it is that makes any cultural artefact endure: its susceptibility to fulfil the different evaluative criteria, and social needs, of different times.