In the Fallout universe, ghouls are humans who received significant amounts of radiation exposure and underwent dramatic physiological mutations. These mutations manifest outwardly in the form of devastated flesh and a "ghoulish" appearance. Biologically, ghouls are immune to and sometimes healed by radiation exposure and have prolonged lifespans relative to typical humans.
Most ghouls were victims of radiation exposure after the war due to the lack of adequate shelter. Ghouls who underwent their mutations as a direct result of the war, otherwise known as pre-war ghouls, are rare but do exist.
Some ghouls, particularly those in New California, were the results of experiments. In Vault 12, located beneath the town of Bakersfield, California, survivors became ghouls after the vault doors were intentionally sealed improperly, allowing radiation to seep into the vault to test its effect on humans.
Ghouls who experience particularly severe mutations become "feral," wherein they lose their mental faculties and become animalistic. Feral ghouls are a significant threat to human settlements due to their numbers and speed. Feral ghoul reavers found In the Capital Wasteland are among the strongest feral ghouls known.
Due to their appearance and association with feral ghouls, ghouls are often the victims of discrimination throughout the Fallout universe. One of the most prominent examples involves Tenpenny Tower in the Capital Wasteland and its residents' refusal to allow a band of ghouls to reside there.