My dissertation takes as its point of departure the recent upheavals in Josquin scholarship. With the composer's date of birth pushed substantially forward, the fading importance of his Milanese period, and dozens of lingering questions with regard to authorship, there is an urgent need to reassess Josquin's compositional development. Crucially, the Sistine Chapel now looks like the place where some of the most exciting developments in Josquin's career occurred, correcting the earlier notion that these took place in Milan. In particular, Josquin composed a series of polyphonic mass settings while in Rome, music so famous that it was published in the first ever single-author music print. But those masses were not composed in a vacuum. While in Rome, Josquin sang newly acquired music from lavish papal manuscripts and worked alongside prominent singer-composers. Through study of Josquin's pre-Roman music, an exhaustive analysis of the papal chapel's repertory, and a stylistic comparison of masses by Josquin and his contemporaries, my dissertation addresses the following questions: Given the new biographical picture, how did Josquin develop as a composer? To what extent was he influenced by the music of his "Roman" contemporaries? And is Josquin's music typical of his generation, or does it stand out from the pack?

I employ newly developed methodologies to assess this complex repertory, focusing on the "surface-level" analysis of contrapuntal construction at the local level. Chief among my findings is the revelation that Josquin's mass music is in fact distinguishable from that of his contemporaries because of his obsessive compositional personality: more than anyone else, Josquin takes fragmentary musical ideas and spreads them densely across the entire texture by means of ostinato and other repetitive devices. Another important finding relates to Josquin's colleagues in Rome, particularly to Marbrianus de Orto (d. 1529). Not only is de Orto's music far more compelling than has been acknowledged, but Josquin seems to have borrowed a number of compositional techniques from de Orto, incorporating them into his Roman masses. Such findings lead to an unfamiliar conclusion: that Josquin engaged with and learned from the music he sang in Rome. They also make it possible for the first time to acknowledge the impressive contributions of composers like de Orto both to the genre of the polyphonic mass and to Josquin's compositional development more generally.