Tadeusz Kotarbiński was one of the main representatives of the Lvov-Warsaw School. The paper presents his life and didactic activity, as well as the list of his main works and the greatest achievements in philosophy. Kotarbiński supported concretism, radical realism, and the directive of semantic reism. He created praxiology—the theory of effective action. He advocated the system of ethics independent of particular worldview (religious worldview including). Kotarbiński influenced, e.g., Stanisław Leśniewski's ideas of ontology (scil. his original logic of names) and of mereology (scil. his original theory of collectives).