This thesis is a study of some major aspects of Martin Heidegger's conception of the nature of man. Central attention is given to Heidegger's major work, Sein und Zeit (1927). In the opening chapters, the context within which the theories presented in this work are to be understood is fixed through successive examinations of some important philosophical concepts. The first chapter looks into the notion of philosophical anthropology, and analyses Heidegger's attitude towards this philosophical discipline and towards the question "What is man?". In the second chapter Heidegger's description of his philosophy as an ontology is discussed, and it is argued that he can appropriately be seen as a thinker standing in the Kantian tradition of transcendental philosophy. In the next chapter Heidegger's links with the existential philosophy of Kierkegaard are discussed, and a general perspective is suggested for an understanding of the tasks of Heidegger's theory of human existence: it is the idea that Heidegger is attempting to present a 'this-worldly' philosophy which nevertheless preserves themes originating in a dualistic and religious mode of thought. The fourth chapter treats a number of aspects of the relationship between Heidegger and his phenomenological predecessor, Edmund Husserl. The problem of reconciling the existential and the ontological aspects of Sein und Zeit is explored here. The next two chapters are designed to supply a basic outline of Heidegger's conception of human existence, setting out the possible interpretations of his notion of existential possibility, and then moving on to look into the distinction between the authentic and inauthentic modes of existence. The seventh, eighth and ninth chapters focus on particular elements within Heidegger's general theory of the human being: the notions of temporality and of Being-towardsdeath, and the question of interpersonal relations. These last two are treated as 'test cases' for judging the adequacy of the Heideggerian concept of human existence. On the one hand, it is seen that Heidegger is prevented from offering any plausible account of the interpersonal sphere by his own fundamental assumptions. On the other hand, however, his theory of Beingtowards-death is defended against the criticisms of a number of his interpreters, and it is argued that this theory provides insights which are lacking in the traditional conception of human mortality. The difference between these findings suggests that it may be impossible to construct a theory of human existence which will be equally adequate to every aspect of the human being.