The origin of the Bengali Muslims became a much debated question after it was found out that they formed the majority of the population of Bengal, especially in the rural areas. This thesis does not tackle the whole problem (origin of the Bengali Muslims); it is primarily concerned with one aspect of it, viewed over a limited period of history which seems peculiarly significant. It starts with the question: how did Islam spread into Bengal during the pre-Mughul period (1204-1538 A.D.)? The attention is focused on Islam as a religious belief, in an attempt to review and assess different factors which may account for its spread in Bengal. Chapter I ("Pre-Islamic Bengal") surveys the past history of Bengal and its condition at the time of the Muslim conquest, in order to suggest a link between some trends of that history and the developments of the Muslim period. Chapter II ("Socio-political factors") summarizes the political history of the Muslim period undar review, pointing out features which form the context of the religious developments; it examines the part played by the immigration of Muslims from other areas, and discusses the situation of the non-Muslim subjects in relation with the phenomenon of conversion to Islam. Chapter III ("Religious factors") tries to describe and assess the prominent role played by the Sufi saints in the spread of Islam in Bengal. Their activities are related to the general background of Sufism in India, to the features of Islâm in Bengal as well as to pre-Islamic history; this suggests a reassessment of the modern idea of "conversion" as applied to the phenomenon recorded in Bengal during that period.