This study investigates an almost unknown musical culture: that of cloistered nuns in one of the major cities of early modern Europe. These women were the most famous musicians of Milan, and the music composed for them opens up a hitherto unstudied musical repertory, which allows insight into the symbolic world of the city. Even more importantly, the music actually composed by four such nuns, Claudia Scossa, Claudia Rusca, Chiara Margarita Cozzollani, and Rosa Giacinta Badalla - reveals the musical expression of women's devotional life. The two centuries' worth of battles over nuns' singing of polyphony, studies here for the first time on the basis of massive archival documentation, also suggest that the implementation of reform in the major centre of post-Tridentine Catholic renewal was far more varied; incomplete, subject to local political pressure and individual interpretation, and short-lived than any religious historian has ever suggested. Other factors that marked nuns' musical lives and creative output - liturgical traditions of the religious orders, the problems of performance practice attendant upon all-female singing ensembles - are here addressed for the first time in the musicological literature.