This book examines the intertwined history of Jewish and Islamic mysticism through a multifaceted study of Egyptian Jewish pietism during the period of Abraham Maimonides (1186–1237) and his circle, from its rise in the late twelfth century through its apex in the mid-thirteenth. The book traces the history and development of the pietist movement, including its origins, scope, communal impact, and far-reaching religious reforms, through an examination of literary manuscripts and documentary fragments from the Cairo Genizah. With a comparative focus on parallel developments in the Islamic sphere, this work analyzes both the inner dynamics and outer matrix of the Jewish pietist movement with its paradoxical strategy of openly adapting Sufi models to bring about a renewal of Jewish religious life. Adapting key rites from their immediate Sufi environment, the pietists cultivated a rigorous regimen of fasting, prayer, and solitary meditation, and organized an intimate fellowship circle of disciples loyal to a common shaikh. Egyptian Jewish pietism is the most explicit phenomenon of Jewish spiritual engagement with its Islamic milieu, and has yet to receive a comprehensive scholarly treatment. This book reveals the profound impact of the pietist movement in its own time and place, and assesses the implications of this remarkable phenomenon in the history of Jewish–Muslim relations.