The In De Ente et Essentia is a polemical work of a brilliant young Dominican professor, Thomas de Vio Cajetan (1469-1534), eager to defend the philosophical legacy of Thomas Aquinas. Drawing from the full range of St. Thomas's writings, the work offers a coherent Thomistic metaphysical theory set against the rival Scotist and Averroist positions defended at Italian Universities in the 1490s. This new English translation is an invitation to philosophers and historians to re-engage with Cajetan, with his 15th Century rivals, and, of course, with Thomas Aquinas himself. It is also a challenge and inspiration to do metaphysics through dialectical engagement with the greatest rival theories of our own time and place.