This chapter identifies the classical philosophical concepts with which existentialism is concerned—being, non-being, and becoming, existence, and essence. It shows how existentialist philosophers transform these abstract ideas to consider the concrete existence of the human individual from a subjective point of view. Starting from Whitman’s recognition of the here and now, and proceeding through Kierkegaard, Nietzsche, Heidegger, Sartre, and Beauvoir, it is demonstrated how traditional philosophical categories first conceived by ancient philosophers echo through the existentialist movement. Kierkegaard’s rejection of idealist rationalism, Nietzsche’s retrieval of Heraclitus’s theory of becoming, Heidegger’s understanding of the human being as Dasein or “being there,” Sartre’s notion of “existence precedes essence,” and Beauvoir’s comparison of existentialist conversion to the phenomenological reduction are discussed in light of existentialist affirmation of the transience and particularity of the human self.