Although Jews were at the centre of commercial activity in medieval Europe, a talmudic ban on any wine touched by a Gentile prevented them from engaging in the lucrative wine trade. Wine was consumed in vast quantities in the Middle Ages, and the banks of the Rhineland hosted some of the finest vineyards in northern Europe. German Jews were, until the thirteenth century, a merchant class. How could they abstain from trading in one of the region's major commodities? In time, they ruled that it was permissible to accept wine in payment of debt, but forbade trading in it, and they maintained that ban throughout the Middle Ages. Further study in the twelfth century, however, led Talmudists to discover that Jews were only forbidden to profit from trading in Gentile wine if they dealt with idolaters, but that trade with Christians and Muslims was permitted. Nevertheless, the German community refused to take advantage of this clear licence. Using Jewish and Gentile sources, this study probes the sources of this powerful taboo. In describing the complex ways in which deeply held cultural values affect Jews' engagement in the economy of the surrounding society, this book also illustrates the law of unintended consequence show the ban on Gentile wine led both to a major Jewish contribution to German viticulture and to the involvement of Jews in moneylending, with all its tragic consequences.