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 Ren 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.