 Nicolai Hartmann (1882–1950) was one of the leading German philosophers of the first half of the twentieth century. Originally a student of Marburg Neo-Kantianism, Hartmann departed from this tradition and spearheaded the resurgence of ontology in the early twentieth century. Hartmann was in many respects a classical philosopher and wrote book-length works scrutinizing and developing all the major fields of philosophy, including the philosophy of history, epistemology, ethics, and aesthetics, although his chief interest was ontology. He held teaching posts in Marburg, Köln, Berlin, and Göttingen, was president of the German Philosophical Association after the second World War, and had a major influence on a generation of German philosophers through his ongoing discussion groups. Hartmann discouraged discipleship by inviting students to think through problems for themselves. Over the last twenty years, after a long period of neglect, his work has been attracting more attention by an international group of scholars. Perhaps unique in the early twentieth century, Hartmann developed a pluralistic, humanistic realism that attempted to do justice to both the sciences and the humanities. Hartmann may be regarded as the first genuine ontological pluralist of the twentieth century.