Existential philosophy has often been identified with moral nihilism and the idea that everything is permitted. This book shows that, in the case of Martin Heidegger, any such assumption seriously distorts the true character of his thought. In Being and Time Heidegger sketches a concept of human coexistence as Mitsein - our being together with one another in the world. He also makes clear that Mitsein has ethical implications, although he failed to develop them in any detail. This study carries forward the analysis of the ethical character of Mitsein and at several points moves significantly beyond Heidegger's account of it. This book is an important statement about the basis of human sociality. It is a major contribution to the continuing debates about Heidegger in particular and ethics in general.