This thesis seeks to study in depth the contribution to Sufism made by the little-known 4th/10th century figure Abū Ṭālib al-Makkī. The first chapter deals with an analysis of the life and works of al-Makkī 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 Kitāb al-tawba of al-Makkī. The fourth chapter assesses the originality of al-Makkī's Kitāb al-tawba and compares it with similar Ṣūfī works of the period. The fifth chapter attempts to compare and contrast al-Makkī and al-Ghazālī's approach to Sufism through an analysis of their respective Kitāb al-tawba. It has long been known that in his work on Sufism, al-Ghazālī depended very heavily upon al-Makkī. This thesis conclusively proves this dependence whilst pointing to the more sophisticated presentation of al-Ghazālī. Al-Makkī is shown to have been one of the early Ṣūfī figures who tried to harmonise the views of the Baṣran and Baghdād Ṣūfī groups and to effect a synthesis between moderate Sufism and the Sharī'a.