This controversial study breaks with received opinion that imagines two distinct religions, Judaism and Islam, interacting in the centuries immediately following the death of Muhammad in the early seventh century. Tradition describes these relations using the trope of “symbiosis.” This book instead argues that various porous groups—neither fully Muslim nor Jewish—exploited a shared terminology to make sense of their social worlds in response to the rapid process of Islamicization. What emerged as normative rabbinic Judaism, and Sunni and Shiʿi Islam were ultimately responses to such marginal groups. Even the development and spread of rabbinic Judaism, especially in the hands of Saadya Gaon (882–942 CE), was articulated Islamically. The emergence of the so-called golden age in places such as Muslim Spain and North Africa continued to see the articulation of this “Islamic” Judaism in the writings of luminaires such as Bahya ibn Paquda, Abraham ibn Ezra, Judah Halevi, and Moses Maimonides. Drawing on social theory, comparative religion, and original sources, this book presents a compelling case for rewriting our understanding of Jews and Muslims in their earliest centuries of interaction. Not content to remain solely in the past, however, it also examines the continued interaction of Muslims and Jews, now reimagined as Palestinians and Israelis, into the present.