 Charles Hartshorne (pronounced Harts-horne) is considered by many philosophers to be one of the most important philosophers of religion and metaphysicians of the twentieth century. Although Hartshorne often criticized the metaphysics of substance found in medieval philosophy, he was very much like medieval thinkers in developing a philosophy that was theocentric. Throughout his career he defended the rationality of theism and for several decades was almost alone in doing so among English-language philosophers. Hartshorne was also one of the thinkers responsible for the rediscovery of St. Anselm’s ontological argument. But his most influential contribution to philosophical theism did not concern arguments for the existence of God, but rather was related to a theory of the actuality of God, i.e., how God exists. In traditional or classical theism, God was seen as the supreme, unchanging being, but in Hartshorne’s process-based or neoclassical conception, God is seen as supreme becoming in which there is a factor of supreme being. That is, we humans change for a while, whereas God always changes, Hartshorne maintains. The neoclassical view of Hartshorne has influenced the way many philosophers understand the concept of God. In fact, a small number of scholars—some philosophers and some theologians—think of him as the greatest metaphysician of the second half of the twentieth century, yet, with a few exceptions to be treated below, his work has had limited influence among analytic philosophers who are theists.