This thesis pays attention to the nature of human being that comes to light in Martin Heidegger's Being and Time. In particular, it attempts to show that his notion of authenticity allows for a distinctive and fruitful conception of ethical responsibility, albeit one that challenges us to rethink ethics and responsibility anew. I claim that if authenticity is ‘owning’ one's existence in a way that is properly fitted to Dasein's ontological way of being (as nonself-identical, ecstatic temporality), this ownership of self will necessarily be the stance of recognizing and responding to that which always already includes a network of relations involving world and others. On such an understanding, genuine existential care for oneself is also care for others in the most originary way possible. Such an ontological picture has been criticized by some commentators as being too formal, insufficiently historical, and lacking genuine mediation – in short, for being ineffectual as a normative force in real-life situations. The main contribution of this thesis is to argue against such an interpretation by showing that Heideggerian authenticity is a properly dialectical concept, capacious enough to account for the legitimate concerns raised by such criticisms, while also being productive for new articulations of what is really normative about human relations.