In The Holy Spirit as Communion, Leon Harris examines the pneumatologies of Colin Gunton and Frank Macchia. For both theologians, the doctrine of the Holy Spirit is foundational to understanding their doctrine of God, Christology, and ecclesiology. Drawing on the theme of communion, The Holy Spirit as Communion expresses the concept that the Holy Spirit is the person who perfects the divine nature and personhood of the Father and Son. It is the Holy Spirit who perfects the eternal communion within the divine Trinity, which is the source of the divine action that also perfects the communion in creation as an expression of the Father’s will through Jesus Christ. The Holy Spirit as Communion explores the essentiality of the Holy Spirit through a unique approach to Spirit Christology: Gunton is represented by a radicalized version of Chalcedon Christology, and Macchia formulates his account through the overarching metaphor of Spirit baptism. Therefore, the doctrine of God, Christology, ecclesiology, and eschatology cannot be construed without a proper account of pneumatology that takes into consideration the eschatological perfecting work of the third person of the Trinity–who perfects creation’s koinonia as a gift from the Father through the grace of Jesus Christ.