In his review of Being and Nothingness, A. J. Ayer observed that “Sartre’s conception of time […] is evidently of cardinal importance to his system” (1945: 25). Over 40 years later, Anthony Manser noted scholars’ neglect of Sartre’s theory of time, remarking that “[t]he chapter on temporality is the hinge on which the argument of L’Être et le néant pivots” (1989: 25). Yet another 30 years later, that theory is still comparatively neglected. 1 Sartre’s view is twofold: temporality is ontologically dependent on consciousness, and consciousness can exist only temporally. In this chapter, I present the argumentative structure by which Sartre establishes these views. Briefly put, Sartre’s argument is that, since time is structured in relations of ontological dependence (“internal relations”), and since such relations can exist only in virtue of consciousness’s activity of nihilation, temporality depends on consciousness. But since consciousness necessarily performs that activity, it also must exist temporally. I also argue in this chapter that Sartre has overlooked two motivations in developing his theory of temporality: first, to found the method of phenomenological ontology; and, second, to show that human freedom, pace Kant, must be situated within the empirical world.