Boethius’s De institutione musica, written in the sixth century C.E., became perhaps the most read music treatise of the Middle Ages, as attested by its presence in over 150 manuscript sources dating from the ninth century through the late fifteenth century. This dissertation explores the diverse uses and interpretations of Boethius’s treatise in the Middle Ages and early Renaissance through its rewriting in physical sources. It begins by examining Boethius’s language in his own historical context and then goes on to survey three medieval modes of re-inscribing Boethius: the writing of gloss, the addition of diagrams, and the compilation of codices. The dissertation demonstrates that Boethius and his late antique contemporaries viewed the singing voice as a verbal medium better understood through grammar than through the discipline of music and that an early medieval reconception of the singing voice may have enabled both music-theoretical treatment of song and the development of plainchant notation. Carolingian theorists, students, and scribes used selective citation and gloss as a way of making Boethius’s theory, which itself had little to say about song, relevant to musical learning that placed a premium on the understanding of plainchant; medieval students in both monastic and scholastic milieus used writing in manuscript margins as a way of contextualizing music in a broader worldview and weighing conflicting opinions from equally venerated authorities, in some cases making such opinions consonant. The project goes on to reconsider the elaborate diagrams of the manuscript tradition as more than textdependent illustrations, also examining their function as instruments of inquiry, objects of meditation, evidence of luxury, and a technology for visually perceiving the aurally imperceptible music of the spheres. Finally, the dissertation explores the ways in which medieval compilers’ packaging of the treatise in certain types of codices, including liberal arts compendia, quadrivium textbooks, music theory anthologies, and complete works of Boethius, shaped the framework in which the treatise was read, even lending different connotations to Boethius’s basic musical terminology.