This article traces the role of the Holy Spirit throughout Augustine's De Trinitate. After situating Augustine's treatise in terms of texts of Athanasius and Basil on the Holy Spirit, it treats the place of the Holy Spirit in his critique of the existing dogmatic terminology and the distinction between the economic and immanent Trinity. In contrast with the dominant Western and Eastern traditions, for Augustine the Holy Spirit comes to be thought of as God in a privileged sense, that is, as the person of the Trinity who is the most proper bearer of certain privileged names of God, most notably love. The notion of the Holy Spirit as eternal ‘gift’ proves to be especially troubling for Augustine, but also especially productive, and the present reading explicates the complex interrelationships that he is forced to develop among the concepts of love, cupiditas, gift, communio and enjoyment. The analysis of the concept of enjoyment in particular leads to the claim that the notion of property or ownership is completely foreign to God and that the Holy Spirit as communio must be thought as ‘gift’ only insofar as it is disruptive of the realm of ownership, that is, the realm of sin. The article finishes, as the De Trinitate does, with the implications of Augustine's treatment of the Holy Spirit for ethics and ecclesiology.