Petrus Alfonsi's Dialogue Against the Jews (ca. 1109) breaks new ground in the history of Christian anti-Jewish polemics. As a recent convert from Judaism, Alfonsi introduced an intimate knowledge of Jewish literature and contemporary practice absent from earlier Christian sources. This knowledge enabled him to attack for the first time the Talmud (or, more broadly, post-biblical Jewish literature) as a source of Jewish error, with arguments drawn from philosophy and theology, astronomy, medicine, and physics. Equally important, Alfonsi's Dialogue contains an extensive anti-Muslim polemic to explain not only why he abandoned Judaism but also why he rejected Islam and chose the Christian faith. For these reasons the Dialogue has been described as the most important anti-Jewish text of the Latin Middle Ages. This assessment is based not only on its innovative argumentation but also on the fact that it was one of the most popular medieval anti-Jewish polemics written. It was cited, often verbatim, by later Christian polemicists like Peter of Blois and used by Peter the Venerable. Alfonsi's Dialogue was known to Joachim of Fiore, who adapted its illustration of the mystery of the Trinity contained in the tetragrammaton; summarized by Vincent of Beauvais, who included a long extract from the Dialogue in his popular Speculum historiale; exploited by Raymund Martini in his monumental Pugio Fidei; and utilized by Abner de Burgos in his Mostrador de Justicia. It was also likely employed by Pablo Christiani to prepare for the public disputation at Barcelona (1263 C.E.) and later by Jerome de Santa Fe for the disputation at Tortosa (1413-1414 C.E.). Never before translated into English, this work presents to the reader perhaps the most important source for an intensifying medieval Christian-Jewish debate.