The present thesis is an attempt to understand Abd Al-Karim Al-Jili's thought and to illustrate his original contribution to the development of medieval Islamic mysticism. In particular, it maintains that far from being an obscure disciple of Ibn Arabi, Al-Jili was able to overcome the apparent contradiction between the doctrinal assumption of a transcendent God and the perception of divine immanence intrinsic in God's relational stance vis-à-vis the created world. To achieve this, this thesis places Al-Jili historically and culturally within the Sufi context of eighth-ninth/fourteenth-fifteenth centuries Persia, describing the world in which he lived and the influence of theological and philosophical traditions on his writings, both from within and without the Islamic world. A whole chapter is dedicated to the definition of the controversies that afflicted Islamic theology and philosophy over the issue of anthropomorphic representations of God and the relevance that this had on the subject of divine immanence and transcendence. Al-Jili's original contribution to this discussion, summarised in the concept of the Perfect Human Being, is illustrated with the editing and translation of one of Al-Jili's works, The Cave and the Inscription, followed by annotations to the book.