This study examines the jurisprudential writings of medieval Rabbanites, Jews in the Islamic world who saw themselves as heirs to the talmudic tradition. Rabbanite Jews were the first to author systematic accounts of talmudic law, which they attempted to transform from an amorphous, dialectical, and discursive corpus into a structured, elegant, and logical system. In so doing, they sought to impose a coherent structure on their legal traditions that would be compatible with larger theological, philosophical, and epistemological ideas. By subjecting Rabbanite legal theory to diachronic and synchronic analysis, this dissertation demonstrates that Rabbanites were involved in a multilayered conversation that engaged their talmudic past, Rabbanite and non-Rabbanite coreligionists, and elements of the Islamic intellectual tradition that were most helpful for the explanation and reconsideration of their own tradition. While Rabbanite legal theory drew heavily on talmudic ideas, it was, at its core, profoundly contemporary, spurred by both Qaraite and Islamic legal theory, among many other factors. This study concentrates on Rabbanite thinking about two, frequently intertwined, topics: the nature and scope of extra-scriptural traditions, known as Oral Torah, and the methodology to be used in enumerating the 613 commandments, which, talmudic legend claims, were given to Moses at Sinai. Acknowledging earlier scholarship on these topics, this study presents a more holistic picture of Rabbanite legal theory. Particular attention is paid to the Judeo-Arabic writings of Moses Maimonides (1138-1204), the Rabbanite author who appears to have been most explicitly concerned with problems of legal theory. Other central figures include Saʿadya ben Joseph Gaon (882-942), Daniel ben Saʿadya ha-Bavli (fl. early thirteenth c.), and Abraham ben Moses Maimonides (1186-1237).