This text endeavours to identify and define the phenomenon of freethinking in medieval Islam, in particular as exemplified in the figures of the two most notorious intellectual heretics, Ibn al-Rawandi (9th-century) and Abu Bakr al-Razi (10th-century). The development of Islamic freethinking is analyzed on the background of the paramount importance of prophetry in Islam. The book examines the image of the freethinkers in Islam and its connection to the legacy of late antiquity, and to the traditions about Indian and Sabian religions. The last chapters examine repercussions of this phenomenon in various aspects of Muslim, Jewish and Christian medieval thought. The author argues that, despite its rare occurrence, freethinking was in fact a pivotal Islamic phenomenon, which had a major impact on the development of Islamic thought.