The undead are beings in mythology, legend, or fiction that are deceased but behave as if alive. Most commonly the term refers to corporeal forms of formerly alive humans, such as mummies, vampires, and zombies, who have been reanimated by supernatural means, technology, or disease. In some cases (for example in Dungeons & Dragons) the term also includes incorporeal forms of the dead, such as ghosts.

The undead are featured in the belief systems of most cultures, and appear in many works of fantasy and horror fiction. The term is also occasionally used for real-life attempts to resurrect the dead with science and technology, from early experiments like Robert E. Cornish's to future sciences such as "chemical brain preservation" and "cryonics."