The first volume of our new series, Fathers of the Church: Shorter Works, will be available in the summer of 2021. This series, to be printed only in paperback format, will offer English translations of treatises, homilies, poems, and letters of the Church Fathers in slim, easily affordable volumes. In this way a multitude of important writings will become accessible to scholars and students as well as the reading public. This is the first complete English translation of St. Gregory of Nyssa’s treatise On the Six Days of Creation (In Hexaemeron). It was probably written in 380-381, and is designed as both a defense and a critique of his recently deceased brother St. Basil’s better known homilies on the creation story as set out in the first chapter of Genesis. At the same time it incorporates Gregory’s own observations on the Genesis text, which reflect his desire to show the consistency between Scripture and the philosophy and natural science of his day. A notable feature is Gregory’s presentation of God’s creation of the world as what has been called a “substantification” of God’s own will, creatio ex Deo rather than creatio ex nihilo . Other ideas of his seem interestingly to foreshadow those of modern science, notably his challenge to the idea that matter is a primary ontological category and his theory that the world as we know it developed through a process of “sequence” ( akolouthia ) from an originally simultaneous creation of everything. Gregory differs from Basil in maintaining that the “waters above the firmament” in Genesis 1 are spiritual rather than physical in nature. He uses a modified form of Aristotle’s theory of elements, together with some interesting observations on geography and meteorology, to construct a detailed and ingenious account of the “water cycle.” This description enables him to refute Basil’s notion that there needs to be an extra supply of physical water above the firmament so that the water lost from earthly seas and rivers through evaporation can be “topped up.”