This chapter describes and discusses the material about the Sibyls as witnesses of Christ's incarnation which circulated separately from the Sibylla Tiburtina during the Middle Ages. This material, which calls the Sibylline Tradition, was pervasive in the medieval period and provided the context in which the Sibylla Tiburtina was understood. It concentrates on the christological aspects of the Sibylline Tradition which intersect with the christological content of the Tiburtina. It is important to note that in fact the Sibylline Tradition could cross-fertilise the Sibylla Tiburtina in other ways too. An interest in the Tiburtina's christological predictions is conceivable not only in the light of the medieval reference mentality and the modern concept of a reader's horizon of expectation, but also in view of the body of extant manuscripts. Curiously, however, modern scholars have never considered whether medieval readers applied the intellectual conventions of the Sibylline Tradition to the interpretation of the Tiburtina.