Nicolai Hartmann was one of the most original twentieth century German philosophers. Yet, he did not compromise clarity and rigor for proficiency and originality. Brought up as a neo-Kantian, he became one of the most important critics of German idealism and one of the most resolute proponents of ontological realism. He developed what he called the “New Ontology,” which lies at the core of a systematic opus branching out in all of the main areas of philosophy. He proposed innovative and promising solutions to old and new problems in the various areas of philosophy. His work had a major influence in philosophy as well as in several sciences. The papers collected in this volume explore themes such as Hartmann’s theory of categories, his aporetics, theory of values and ethics, philosophy of biology, philosophy of mind, philosophy of mathematics, the influence he had on anthropology, and his place in the history of philosophy.