The Abbasid caliph al-Ma'mun (786-813) is best known for sponsoring the translation of Greek philosophy into Arabic and for promoting the activity of mathematicians, astronomers, engineers, physicians and cartographers. In his own time, however, he was a wildly controversial figure: having come to power by overthrowing the central government in Baghdad and killing his half brother, the caliph al-Amin, he astonished his contemporaries by choosing a member of the rival house of Ali as his heir apparent. A self-proclaimed defender of the faith, he supported theological positions many believers considered heretical, and towards the end of his career launched an inquisition against the scholars of Hadith who refused to accept his guidance on matters of theology. This new biography offers an insight into the life and times of al- Ma'mum, treating him as the product of an age that has come to be seen as a formative period in the development of Islamic law and theology. Drawing on both contemporary and well-established sources, it offers a comprehensive portrait of one of the most fascinating figures in Islamic history, and will appeal to readers of all backgrounds.