Cryonics is the low-temperature preservation of humans and other animals that can no longer be sustained by contemporary medicine until resuscitation may be possible in the future. Human cryopreservation is not currently reversible. In the United States, cryonics can only be legally performed on humans after pronounced legally dead. The rationale for cryonics is that the process may be reversible in the future if performed soon enough, and that cryopreserved people are not dead by the modern information-theoretic definition of death. Cryonics is derived from the Greek word κρύος (kryos), meaning cold.

There is a high representation of scientists among cryonics supporters. Scientific support for cryonics is based on studies showing substantial preservation of brain cell structure by current methods, and projections of future technology, especially molecular nanotechnology and nanomedicine. Some scientists believe that future medicine will enable molecular-level repair and regeneration of damaged tissues and organs decades or centuries in the future. Disease and aging are also assumed to be reversible. Many ethical questions revolve around the issue of whether cryonics can work.

The modern concept of cryonics as a general procedure to apply whenever patients are considered beyond help by the medicine of their time was originated in 1962 by Robert Ettinger. The largest current practitioners of cryonics are two member-owned, non-profit organizations, the Alcor Life Extension Foundation in Scottsdale, Arizona, with 77 cryopreserved patients and 842 members, and the Cryonics Institute in Clinton Township, Michigan with 86 patients and 718 members. In Europe, the Russian company KrioRus (founded 2005) maintains four cryopreserved patients in Moscow.