A distinction between Gregorian chant and various repertories of "medieval chant" is generally accepted. One of the most important, but least studied, of these medieval repertories is that of the Kyrie. This chapter attempts to illuminate those features of the early Kyrie that give it its distinctive character and set it apart not only from Gregorian chant but also from other types of medieval chant. According to a traditional interpretation, the history of the Kyrie within Christian liturgy—insofar as it can be documented—began as a response in a long series of petitions known as "litanies." The earliest sources for Kyrie melodies, as for medieval chant generally, are manuscripts written in Frankish territory during the tenth century. The importance of the Aquitanian manuscript tradition rests largely on one other factor, namely the legibility of the musical notation. For over a century, scholars have scrutinized the tenth- and eleventh-century Aquitanian manuscripts that are repositories for so much medieval chant.