This first book-length study of Paul Tillich’s ethics is drawn from research in the Harvard Archives of Tillich and fifty years of teaching Tillich’s social-political thought. Ronald H. Stone, a past president of the North American Paul Tillich Society, dialogued about Tillich’s social ethics with Communists in East Berlin, Buddhists in Bhutan, Hindus’ in Madurai, Jews in Jerusalem, and Muslims in Damascus. In this his fourth work on Tillich’s philosophy the ethic is examined from the early ontological to socialist ethics to his own final principled-situationalist ethic in late life. Unique to this study is the in-depth inquiry into Tillich’s courageous social action correlated with his own philosophical-theological ethic. The book moves from an early socialist rally in Berlin, through the wars, dialogue with John Foster Dulles about post-war planning, debates about nuclear deterrence, to Buddhist Christian dialogue. The author’s own preference for the late ethic of the philosopher informs the inquiries into the earlier radical Tillich. The conclusion provides a synthesis of the vast sources of Tillich’s ethics and presents twelve themes summarizing sources and future resources for ethics from his life’s work. Tillich’s ethic is a model for how much Christian theology needs to rediscover its ethic and social relevance.