The history of asceticism and mysticism in the Islamic tradition, with particular attention to the formative period as well as the breadth and diversity of Sufism across the Islamic world, from West Africa and Spain, to the Balkans and Central Asia, to China and Southeast Asia. We begin with the origins of the piety movement in early Islam, and the ways in which it responded to late antique Christian monasticism. We then examine the emergence of classical Sufism in the 9th century, its basic concepts and practices, the rise of saints, the formation of Sufi orders, the social and cultural context for key developments, as well as the reasons for the contested legacy of Sufism in the modern world. Major figures for discussion include Rabi'a, Junayd, Hallaj, Ghazali, Ibn Arabi, and Rumi.