Isaac ben Samuel of Dampierre, often known as Ri ha-Zaken, was a leading figure in the Tosafist movement. The Tosafists were active in Northern France during the twelfth and thirteenth centuries. Their dialectical approach to interpretation of the Talmud and other classic rabbinic texts created a revolution in Jewish intellectual life. This volume gathers, for the first time, 140 responsa written by Isaac of Dampierre in response to questions of practical law and legal interpretation. Each responsum was edited according to the most accurate manuscripts, with a critical apparatus and extensive historical and textual notes. The introduction explores Isaac of Dampierre’s life and thought as they emerge from the responsa.