In this short article, I will try to explain the concept of being-in-the-world of the German philosopher Martin Heidegger in his major work Being and Time (Ger . Sein und Zeit). But above all, to know his point of view on Being-in-the-World and the phenomenological tradition in philosophy, it seems necessary to me to know the general structure of his philosophy. In everyday language, we often mention the word “Be” in different situations. But, generally, by Being, they mean being. But what is Being? Is it a totality of beings or a method for understanding beings? However, since Being is not a being, it is not possible to understand beings using Being. Furthermore, because being is neither a being nor a thing, it is neither temporal nor in time. The relationship between Being and Time must be sought elsewhere. To investigate this, Heidegger wrote his masterpiece Being and Time (Sein und Zeit). Heidegger claimed that the answer to the question of “being” can be found via a fundamental ontology that he created. It is an ontology from which each ontology arises. The subject of this fundamental ontology is an ontologically unique being: Dasein or the life of the human being. What makes Dasein ontologically unique is that it is the only being that attempts to understand its own being during its life. Dasein literally means “to be there” (Da – Sein). However, in Heidegger's conception, “Da” means clarity more than there. Heidegger uses Dasein in his everyday use, but he also means something else by using Dasein. "Heidegger follows everyday usage in this respect (Dasein meaning the being which belongs to persons), but he goes a little further in that he...... middle of paper ...... nites, 1) being, 2) facticity, 3) fall. These three phenomena correspond to these three ecstasies (All. Ekstase) of time: a) future (All. Zukunft), b) having-been (All. Gewesenheit), c) lasting as presence (All. Gegenwart). What establishes the essence of time is the union of these three ecstasies. Heidegger, who sought to establish a fundamental ontology, in his unfinished work Being and Time, studied the existential characteristics of Dasein and the relationship between Dasein and temporality. As Heidegger shows, investigating the human always means investigating the question of being. In order to investigate the human being or Da-sein, it is obligatory to answer the question of Being. The point Heidegger wanted to reach in Being and Time could be summed up in his own words: “Dasein is not in time, but time itself” (Heidegger, 85).