The question ‘Who is Hindu, who is Muslim?’ is addressed here and found to be not as simple as generally assumed. By analysing documents as well as original field data, the author examines the shaping of religious identities in South Asia, and more particularly in northern India. She argues that the perception of Islam and Hinduism as two monolithic and perpetually antagonistic faiths coexisting uneasily in South Asia has become so deeply ingrained that the complexity of the historical fabric is often neglected. The emergence of clear-cut categories is demonstrated to be a relatively recent phenomenon, while the past is shown to have been characterised by a remarkable fluidity and diversity in the social and religious milieus. The author explores the historical mechanisms that have led to the emergence and crystallisation of religious identities, and the increasing number of conflicts that threaten the harmonious coexistence of contemporary South Asian communities. In particular, she considers the role played by Ismaili Islam in the intricate interface of South Asian religious traditions, which has to this day remained largely unexplored.