This book focuses upon St Thomas Aquinas's much neglected proof for the existence of God in De Ente et Essentia Chapter 4. It offers both a contemporary presentation and interpretation of this proof and also a defense. Beginning with the distinction between essence and esse in Aquinas's thought, the book moves from an account of these metaphysical principles to their use by Thomas in establishing that there is a single unique primary cause from which all that is comes to be. Along the way, important themes in metaphysics, such as (i) essence, (ii) existence, (iii) causality, (iv) causal infinities are addressed, dealt with, and defended within a Thomistic perspective. The outlook of the book is not parochial, but engages contemporary authors who either (i) misinterpret or (ii) disagree with Aquinas. By the end it is established that Aquinas is on firmer ground than all of his detractors, and that the success of this proof of God has ramifications for how we think about the nature of creation. This book is intended to fill a lacuna in the Thomistic studies literature which until now has not given the proof from the De Ente et Essentia the attention it deserves.