Usul al-Fiqh, often inaccurately translated as "principles of jurisprudence," is a discipline within Islamic studies that serves as the methodology for deriving legal rulings. Shia scholars have, in the last two centuries, refined this field into an unmatched intellectual system-a logical framework that not only functions as the foundation of jurisprudential reasoning but also stands as an independent science addressing broader hermeneutical challenges. This book is the first of its kind to systematically define the key terminologies of Usul al-Fiqh in Part One, providing a foundation for the subsequent in-depth discussions. Part Two explores the intellectual implications of these concepts. Part Three addresses issues concerning hujjiyyah (authority) within the jurisprudential framework, grounding these in the four foundational sources of Shia jurisprudence: the Holy Quran, the sayings and actions of the inerrant Ahlul Bayt (peace be upon them) (sunnah), rational deduction ('aql), and consensus (ijma') as well as other lesser-known ones. Finally, Part Four examines al-Usul al-'Amaliyya, the practical principles applied in deriving rulings. This book is a valuable resource for those seeking to understand the foundations of Islamic jurisprudence, both from within the tradition and from an academic or interfaith perspective. It offers a detailed exploration of the methodologies employed in Shia scholarship, making it accessible to both scholars and those interested in Islamic legal theory.