Heidegger's concept of 'uncanniness' (Unheimlichkeit) is usually overlooked, glossed over, or conflated with Freud's uncanny feeling. This dissertation establishes the ontological significance of uncanniness and its importance in Heidegger's project by showing that 'uncanniness' does not name a feeling or any affective phenomenon, but is one of Heidegger's words for the ground of Dasein's being. Uncanniness is the condition of possibility of an openness to being or intelligibility -- what has traditionally been called the origin of consciousness. Analysis of uncanniness thus answers the question of how there can be entities like us.;Despite their different vocabularies, Heidegger's three principal discussions of uncanniness present a single picture of it. To show this, I interpret Heidegger's Angst analysis in Being and Time and his two readings of the second choral ode from Sophocles' Antigone (in Introduction to Metaphysics and Holderlin's Hymn 'The Ister') as narrative expressions of the conditions of possibility of openness. In each of these texts, to be uncanny is to 'come to be' or be grounded in a self-enabling reciprocity of openness and finitude. This phenomenon is appropriately termed 'uncanniness' because it shares the uncanny feeling's (Freudian) structure of an interplay between revelation (openness) and hiddenness (finitude). The difference is that for Heidegger this structure determines not our experience but our essence. Uncanniness is a matter not of feeling uncanny, but of being uncanny.