Gregory of Nyssa is widely regarded as the most substantial thinker and theologian among the three Cappadocians and is often used as the representative of Greek trinitarian theology. Through a fresh examination of Gregory's trinitarian theology in its historical context, Michel René Barnes reveals the special importance of the concept of ""power""--dynamis. The book also brings to light the way in which ""power"" served as a fundamental concept in all stages of patristic trinitarian theology and shows the origins of the concept in Greek philosophy and medicine. The most important senses of ""power"" have little to do with political or physical power. Since the time of the pre-Socratics, Greek philosophy and medicine used the term to refer to the capacity of anything to be a cause. It is this sense of ""power"" that figures so substantially in the development of the doctrine of the Trinity in the patristic period. Indeed, what is recognized as the definitive form of Nicene trinitarian theology--normative for all Christians--was established in the second half of the fourth century using arguments based on an understanding of God as a cause--God as a ""power."" Nowhere is the importance of ""power"" for supporting a Nicene trinitarian theology as great and as closely argued as it is in the theology of Gregory of Nyssa. This study will be useful for those who study the development of the doctrine of the Trinity, as well as those who are interested in the role of scriptural and philosophical resources in Christian theology. Finally, the in-depth account of the importance of the philosophical use of ""power"" will be of use to scholars of ancient and hellenistic Greek philosophy.