In the Islamic theory of knowledge, reports about past events are accepted as the most important source of religious knowledge. They are classified as mutawātir and āhād according to their epistemic value. Mutawātir reports are used by theologians and jurists to establish the truth of prophecy and the validity of all transmitted knowledge. This category of reports was not compiled in specific collections by the hadīth scholars, since they were widely known and commonly accepted as sound. Outside the mutawātir category, however, hadīth scholars did collect the āhād hadīths that serve as the basis of religious practice. Beginning in the 9th/15th century, some hadīth experts tried to apply the usūlī understanding of tawātur to hadīths that were considered to be āhād. These new attempts led to some vagueness and ambiguities in the scholarly understanding of the concept. In this essay, I argue that the classification of reports as either mutawātir or āhād began as an epistemological exercise in the fields of theology (kalām) and legal methodology (usūl al-fiqh); and I outline the process by which the concept of mutawātir was applied to the hadīth corpus and the problems to which this gave rise.