Being and Nothingness: An Essay on Phenomenological Ontology (French: L'Être et le néant : Essai d'ontologie phénoménologique), sometimes published with the subtitle A Phenomenological Essay on Ontology, is a 1943 book by the philosopher Jean-Paul Sartre. In the book, Sartre develops a philosophical account in support of his existentialism, dealing with topics such as consciousness, perception, social philosophy, self-deception, the existence of "nothingness", psychoanalysis, and the question of free will.

While a prisoner of war in 1940 and 1941, Sartre read Martin Heidegger's Being and Time (1927), which uses the method of Husserlian phenomenology as a lens for examining ontology. Sartre attributed the course of his own philosophical inquiries to his exposure to this work. Though influenced by Heidegger, Sartre was profoundly skeptical of any measure by which humanity could achieve a kind of personal state of fulfillment comparable to the hypothetical Heideggerian "re-encounter with Being". In Sartre's account, man is a creature haunted by a vision of "completion" (what Sartre calls the ens causa sui, meaning literally "a being that causes itself"), which many religions and philosophers identify as God. Born into the material reality of one's body, in a material universe, one finds oneself inserted into being. In accordance with Husserl's notion that consciousness can only exist as consciousness of something, Sartre develops the idea that there can be no form of self that is "hidden" inside consciousness. On these grounds, Sartre goes on to offer a philosophical critique of Sigmund Freud's theories, based on the claim that consciousness is essentially self-conscious.

Being and Nothingness is regarded as both the most important non-fiction expression of Sartre's existentialism and his most influential philosophical work, original despite its debt to Heidegger. Many have praised the book's central notion that "existence precedes essence", its introduction of the concept of bad faith, and its exploration of "nothingness", as well as its novel contributions to the philosophy of sex. However, the book has been criticized for its abstruseness and for its treatment of Freud.