Tawatur is the concept that information yields certainty if acquired through a sufficient number of independent channels. Tawatur in Islamic Thought is an attempt to unravel the twisted historical threads of the conception and usage of tawatur across diverse Islamic disciplines, in light of both Western academia and debates within Muslim scholarship. In the process, numerous salient questions in Islamic thought are tackled, such as epistemic certitude, scholarly consensus (ijma?), and the rationalism–traditionalism relationship. The study culminates in the question of the extent to which tawatur was used by Muslim scholars to define the boundaries of Islam and of orthodoxy. Tawatur in Islamic Thought shows that the majority voices in Muslim scholarship, across sectarian boundaries, reached a steady-state conception of a two-tiered orthodoxy, corresponding to two tiers of tawatur – an outer tier that includes all who affirm a definitive kernel of Islam and an inner tier that is more exclusive.