This dissertation examines contemporary Islamic debates about the ethics and legality of Sufism (Islamic mysticism) within a globally influential movement based at the Islamic seminary (madrasa) known as Dar al-Ulum Deoband in northern India. Two overarching, and interrelated, questions motivate this dissertation. What are the historical origins of Deoband's critique of Sufism? And how is this critique variously appropriated and contested in the contemporary world? To this end, it reconstructs the contours of Deoband's engagement with Sufism in the works of its founders and how this critique traveled through the Deobandi network to South Africa, home to the most prominent Deobandi madrasas outside of South Asia and wide participation in the Sufi devotions that the Deobandis have most vociferously critiqued. The first and second chapters explore how Deobandis conceived Sufism exclusively as ethical reform (islah) of the self and argued certain devotional practices (especially celebration of the Prophet Muhammad's birthday and of Sufi saints' death anniversaries) are illicit innovations (bidà) in Islam. The third chapter reconstructs how Deobandi thought and institutions took root in South Africa, attending to the circulation of Deobandi writings within South Africa and the founding of South African Deobandi madrasas. The fourth chapter demonstrates how Deobandi students position themselves within an imagined Deobandi network and how they seek to embody and internalize the ethics of Sufi practice. The fifth and sixth chapters assess the reception of Deobandi critiques of Sufism in the South African public sphere, particularly in the context of Muslim politics, society and popular media during and after apartheid