eminent rabbinical scholar who proposed a far-reaching series of legal enactments (taqqanot) that profoundly molded the social institutions of medieval European Jewry.

He was called the light of the exile and also Rabbenu (“Our Teacher,” a title of reverence). As head of the rabbinic academy at Mainz, he was a pioneer in bringing the learning of the Talmudic academies at Babylon and Palestine to western European schools. At synods of community leaders he proposed his taqqanot, which included the prohibition of polygamy (permitted by biblical and Talmudic law but already mostly unpracticed), interdiction of the husband's right to divorce without the wife's consent, prohibition of reading another's mail without his consent (mail then was usually carried by travelers), and prohibition against taunting Jews who had been forcibly converted to another religion and had then returned to Judaism.

He wrote many responsa (authoritative answers in response to questions about Jewish law), worked on a critical text of the Talmud and the Masora, and transmitted to his students an extensive oral commentary on the Talmud. All subsequent rabbinic students in western Europe considered themselves, in the words of the renowned medieval French Jewish commentator Rashi (1040–1105), “students of his students.”