Jan Łukasiewicz was a leading figure of the Warsaw School of Logic—one of the branches of the Lvov-Warsaw School. The paper presents his personality, life and didactic activity, as well as the list of his main works and the greatest achievements in logic, its history and philosophy. In propositional logic, he invented the bracketless symbolism and constructed many systems of this logic. One of his greatest achievements was discovering three-valued logic. He also reconstruct the history of logic with the apparatus of modern logic. In ontology, Łukasiewicz made a logical analysis of the notion of causality and determinism. In epistemology, he established the precise formulation of the paradox of the liar.