This detailed and clearly written book is an invaluable window onto a period of Jewish history that has remained largely unknown to all but a handful of specialists. For more than six centuries two important institutions of Jewish learning and leadership dominated Babylonia, a loose geographic term used by Jews to refer to an area roughly corresponding to modern-day Iraq. From the middle of the 6th to the middle of the 11th century, the heads of these yeshivot (s. yeshivah), known as geonim (s. gaon), exercised a combination of spiritual and political authority over Jewish communities throughout the Near East, North Africa, and Europe. Their most enduring impact on Jewish civilization, however, was the canonization of the Babylonian Talmud, which, as a result of their efforts, became the cornerstone of all forms of medieval rabbinic Judaism. Brody's book, based on a mastery of the primary sources as well as recent work in the field, provides the first comprehensive summary of the achievements of the geonim in almost fifty years, a task made both challenging and imperative by the progress of research on materials from the Cairo Genizah since the publication of S. Assaf's Tequfat ha-geءonim ve-sifrutah in 1955.