Being and Time is the 1927 magnum opus of German philosopher Martin Heidegger and a key document of existentialism. Being and Time is among the most influential texts of 20th century philosophy. It had a notable impact on subsequent philosophy, literary theory and many other fields. Though controversial, its stature in intellectual history has been compared with works by Kant and Hegel. The book attempts to revive ontology through an analysis of Dasein, or "being-in-the-world." It is also noted for an array of neologisms and complex language, as well as an extended treatment of "authenticity" as a means to grasp and confront the unique and finite possibilities of the individual.

Martin Heidegger was a German philosopher who is best known for contributions to phenomenology, hermeneutics, and existentialism. He is often considered to be among the most important and influential philosophers of the 20th century.