[Revised entry by David I. Beaver, Bart Geurts, and Kristie Denlinger on January 7, 2021. Changes to: Main text, Bibliography, notes.html] We discuss presupposition, the phenomenon whereby speakers mark linguistically information as being taken for granted, rather than being part of the main propositional content of a speech act. Expressions and constructions carrying presuppositions are called "presupposition triggers", forming a large class including definites and factive verbs. The article first introduces a sample of triggers, the basic properties of presuppositions such as projection and cancellability, and the diagnostic tests used to identify them....