This chapter discusses the interaction between Judaism, Christianity, and Islam in medieval Europe. It considers the importance of Augustine’s doctrine of Jews as ‘witnesses’ to Christian truth in the formation of the medieval image of the ‘hermeneutical Jew’. Jews, who lived primarily in the Islamic world in the first millennium, began to migrate into Christian lands in greater numbers from the eleventh century. As Christian ideas about Judaism evolved in the twelfth century, culminating in the Fourth Lateran Council of 1215, Jewish authors responded with detailed critiques of Christian belief. The simultaneous Christian engagement with Muslim sources led to a triangular encounter, especially significant in the Iberian Peninsula, between Jewish, Christian, and Muslim writers, reflected in numerous dialogues and polemics about prophecy and history. Beginning in the thirteenth century, mendicant friars, including converts, played a greater role in engagement with Islam and Judaism, taking on important roles as translators and inquisitors.