This Element examines a central concept in Heidegger's philosophy – being-in-the-world – one of his most fundamental characterizations of our (Dasein's) way of being. Central is an exegetical and interpretive account of the concept as it figures in Division One of Being and Time, as well as an assessment of its significance for reconfiguring our understanding of traditional philosophical problems and positions, most notably skepticism, realism, and idealism. Considerable attention is also paid both to the emergence of this concept in Heidegger's lecture courses in the years leading up to Being and Time and to his reflections on it in the immediate aftermath of its publication in both writings and lectures.