Ghosts and spirits are the most frequently encountered supernatural phenomena. They are incorporeal — lacking physical bodies — and are typically the lingering essences of deceased humans. While reapers are not ghosts in the traditional sense, they operate in the same incorporeal realm and interact with the same spiritual mechanics.
Why Ghosts Stay
Under normal circumstances, a person dies and a reaper escorts their soul to the afterlife. Ghosts are the exceptions — souls that, for various reasons, refuse to go or are unable to go. Reasons include: unresolved trauma, desire for revenge, an inability to accept their own death, a strong emotional attachment to a person or place, or interference by external forces (binding spells, curses, or disruption of the reaper who was meant to collect them).
The Deterioration Problem
One of the most important aspects of ghost lore in Supernatural is that ghosts deteriorate over time. A recently deceased person's ghost may be confused but essentially rational. Over years and decades, most ghosts become increasingly fixated, confused, angry, and violent. They lose perspective and nuance, becoming defined by whatever emotion or purpose is keeping them tethered. A ghost that started as a protective mother may become a violent, indiscriminate attacker after enough time passes. This is why the salt-and-burn is considered a mercy as much as a necessity.
Notable Hauntings
The show features dozens of ghost episodes across fifteen seasons. Some of the most memorable include "Roadkill" (Season 2), where the twist revealed the person the brothers were helping was herself a ghost; "Ghostfacers" (Season 3), presented as found footage from amateur ghost hunters; and "Death Takes a Holiday" (Season 4), where Sam and Dean became incorporeal to interact directly with the ghost world.