The eleventh and twelfth centuries represent a critical formative period for institutions and practices that characterized later Islam. Sufism also emerged during the same period as a distinct mode of piety that gained widespread acceptance in the aftermath of Mongol invasions in the thirteenth century. Using early Sufi biographies produced in Khorasan during that period, this study will argue that the early Sufis were not only preoccupied with locating their own tradition within the Islamic orthodoxy but also defining the contours of what constituted acceptable Islam. The sources used are predominantly Persian Sufi biographies composed in Khorasan which form the main body of historiography of Sufism.