How can we understand the phenomena of loss and mourning in the Heideggerian framework? There is no established interpretation of Heidegger that gives an elaborate account of the phenomena of loss and mourning, let alone gauges its importance for our understanding and assessment of authentic existence in Heidegger. This paper attempts to do both. First, I give a detailed exposition of Heidegger’s analysis of the phenomena of mourning and loss and show that Heidegger’s analysis of mourning in his early and (...) late work is strikingly in line with his collectivist understanding of Being-with. This demonstrates, contrary to what some of his proponents argue, that Heidegger does not follow the kind of dynamic understanding of Being-with that places the other within fine-grained spaces of possibility. Second, with reference to Heidegger’s existential philosophy, I construct a phenomenology of mourning and grief. Though Heidegger himself fails to explain the relationships in which one mourns after a close other, we can develop a unique phenomenology of mourning with reference to Heidegger, which shows that each loss is singular and can be equiprimordial with one’s own death in opening one to the possibility of an authentic existence. In this new understanding of authenticity, loss is regarded as a powerful force, akin to death, in leading one toward their self-owned existence. (shrink)