This thesis seeks to study in depth the contribution to Sufism made by the little-known 4th/10th century figure Abu Talib al-Makki. The first chapter deals with an analysis of the life and works of al-Makki against the background of the religious milieu of his time. A major section of the thesis (the second and third chapters) provides for the first time in English a translation of the Kitab al-tawba of al-Makki. The fourth chapter assesses the originality of al-Makki's Kitab al-tawba and compares it with similar Sufi works of the period. The fifth chapter attempts to compare and contrast al-Makki and al-Ghazali's approach to Sufism through an analysis of their respective Kitab al-tawba. It has long been known that in his work on Sufism, al-Ghazali depended very heavily upon al-Makki. This thesis conclusively proves this dependence whilst pointing to the more sophisticated presentation of al-Ghazali. Al-Makki is shown to have been one of the early Sufi figures who tried to harmonise the views of the Basran and Baghdad Sufi groups and to effect a synthesis between moderate Sufism and the Shari'a.