Compared with his Christology, Cyril of Alexandria's pneumatology has received relatively scant attention from scholars in the past one hundred years. Nevertheless, when one consults Cyril's works, especially his biblical commentaries, one finds frequent mention of the Spirit, reflecting deep thought on the subject. Through analysis of his voluminous writings (ten volumes in Migne's Patrologia Graeca), but with special attention to his exegetical works, this dissertation focuses on three neglected aspects of Cyril's pneumatology: his thought on the work of the Holy Spirit with regard to Christian unity, the Bible and its interpretation, and the Christian sacraments. Because Cyril did not write a treatise dedicated to the Holy Spirit (unlike some other ancient patristic figures), the dissertation is essentially synthetic in its methodology. The dissertation argues that Cyril ascribes a significant role to the Holy Spirit in all three areas and in a variety of ways, but that in all three the Spirit plays a rather hidden or subtle role. Further, in terms of Christian unity, Cyril shares his most extensive reflections in his commentary on the Gospel of John, and his ideas seem to be somewhat unusual among his contemporaries. In regard to the Bible, he expresses the Spirit's role using three primary literary devices, but his biggest contribution seems to be his contention that the Holy Spirit was guiding the council participants at the council of Nicaea. In regard to the sacraments, he sees the Spirit working with the other Persons of the Trinity in both baptism and the Eucharist (if most obviously the former), and possibly also in ordination. One of the most significant results of the study is that, for Cyril, pneumatology is always part of a larger Trinitarian structure, even though he can speak quite clearly and extensively about the Holy Spirit specifically. For Cyril, reflection on God the Holy Spirit does not exist apart from reflection on God the Son and God the Father.