This book explores the possibility of using the twentieth-century "process" philosophy of Alfred North Whitehead to modernize the thirteenth-century metaphysics of St. Thomas Aquinas in order to make better philosophical sense of the evolutionary processes of the world. Due to certain limitations, neither philosophy has been able to provide satisfactory metaphysical accounts of the world. In joining the two, these individual limitations are avoided, and the advantages of each--Thomistic metaphysics with its deeper ontology, Whiteheadian metaphysics with its ability to account for the evolutionary advances now apparent in the universe--provide a revised theory that is a kind of "process-enriched Thomism."