ENThis paper discusses the meaning of the notion Faktizität, which Heidegger developed at the start of his career. The meaning of this notion for the rest of the Heidegger’s career is revealed. It is concluded that this notion remained a philosophical mystery and the main task of Heidegger’s thinking was to disclose that mystery. Key words: Faktizität, Dasein, Sein, Martin Heidegger. [From the publication]