Being and Time (German: Sein und Zeit, 1927) is a book by the German philosopher Martin Heidegger. Although written quickly, and despite the fact that Heidegger never completed the project outlined in the introduction, it remains his most important work and has profoundly influenced 20th-century philosophy, particularly existentialism, hermeneutics and deconstruction.

Being and Time was originally intended to consist of two major parts, each part consisting of three divisions. Heidegger was forced to prepare the book for publication when he had completed only the first two divisions of part one. The remaining divisions planned for Being and Time (particularly the divisions on time and being, Kant, and Aristotle) were never published, although in many respects they are addressed in one form or another in Heidegger's other works. In terms of structure, Being and Time remains as it was when it first appeared in print; it consists of the lengthy two-part introduction, followed by Division One, the "Preparatory Fundamental Analysis of Dasein," and Division Two, "Dasein and Temporality."

Martin Heidegger (September 26, 1889 – May 26, 1976); German pronunciation: [ˈmaɐ̯tiːn ˈhaɪdɛɡɐ]) was a German philosopher known for his existential and phenomenological explorations of the "question of Being."

Heidegger argues that philosophy is preoccupied with what exists and has forgotten the question of the "ground" of being. We find ourselves "always already" fallen into a world that already existed; but he insists that we have forgotten the basic question of what being itself is. This question defines our central nature. He argues that we are practical agents, caring and concerned about our projects in the world, and allowing it to reveal, or "unconceal" itself to us. He also says that our manipulation of reality is often harmful and hides our true being as essentially limited participants, not masters, of the world which we discover.

Heidegger wrote about these issues in his best-known book, Being and Time (1927), which is considered to be one of the most important philosophical works of the 20th century. Heidegger's influence is far reaching, from philosophy to deconstructionism and literary theory, theology, architecture, and artificial intelligence.