In 1922 Charles Hartshorne, then an aspiring young philosopher, wrote to Edgar Sheffield Brightman, a preeminent philosopher of religion and one of the earliest members of the Boston School of Personalism. For twenty-three subsequent years, the two carried out an unusually rich and intensive correspondence, and, remarkably, almost every letter was preserved. They are presented here along with additional material that follows the philosophers' lives and interactions after 1945, when Brightman's ill health prevented him from continuing the correspondence. Hartshorne (1897-) has been called ""the world's greatest living metaphysician."" But when the correspondence began, he was just a graduate student, while Brightman (1884-1953) was already an influential philosopher and theologian. Over time, as Hartshorne gained prominence, the letters reveal first a relationship of equals and eventually a reversal of roles as the younger man began to influence his former mentor. Hartshorne's sustained critique of Brightman's epistemological and metaphysical position eventually led to important shifts in Brightman's views. In their introductory essays, editors Randall Auxier and Mark Davies place the correspondence in its intellectual context and address the relationship between personalism and process philosophy/theology in metaphysics, epistemology, ethics, and social philosophy. Theologians and philosophers in a wide range of specialties will welcome this record of an enduring intellectual friendship.