This thesis examines the possibilities for feminism that arise from the work of Michel Foucault, which I explicate by comparison it with humanist existentialism. I begin with The Second Sex, Simone de Beauvoir's application of existentialism to women. I expose the problems that arise in Beauvoir's project. Woman's body is an obstacle to her transcendence, and further, she must abandon her feminine desires and values, and accommodate herself to masculine patterns if she is to overcome her immanence and subordination. To understand why such problems recur in The Second Sex, I turn to Sartre's Being and Nothingness. After examining the conceptions underlying his thought, I conclude that his philosophy is unable to encompass difference, and is therefore antithetical to the feminist project. ¶ Foucault's philosophy offers solutions to these problems by eliminating consciousness as universal subject of action, and by making subjectivity a product of time, through showing how subjects are formed though the changing effects of power upon bodies. His thought encompasses difference at a fundamental level, through understanding human beings as particular 'events' in time. I argue that Foucault's philosophy does not depend fundamentally, as does Sartre's, upon woman as Other. ¶ Foucault shows how our particular historical form of rationality, created within power relations, sets limits on what we can think, be and do. He shows how thought can overcome some of these limits, allowing us to become authors of our own actions. Misunderstandings are common, particularly of his conception of power and its relation to subjectivity. Many commentators demand changes that reinstate the concepts he fundamentally rejects. Others do not see the unity of his philosophy. I show its importance to women's emancipation and to a feminist ethics. ¶ Finally, I compare Foucault's thought with feminism of difference. With the help of Heidegger, I argue that Foucault offers a superior but complementary way to know who we are, through understanding the history of our making. I show how the masculine and the feminine can be reconciled through a reconceptualisation of the relation of sex to time. All told, Foucault is a philosopher of freedom and for him the practice of freedom is an ethics.