Jewish and Islamic legal systems separately have attracted wide interest from scholars, but the relationship between them is not a well-researched area of study. Beginning with Abraham Geiger’s publication of Was hat Mohammed aus dem Judentume aufgenommen? in 1833, scholars in Western academe have observed parallel points between Judaism and Islam, interpreting the relationship in terms of the latter (i.e. Islam) borrowing from the earlier. In the present volume, Gideon Libson challenges this long-established view of the relationship and examines bilateral influence of Jewish and Islamic law in the geonic period (7th – 11th century) in the case of custom, concluding that Islamic law has affected Jewish legal practice in certain aspects.