Tessa: Well, like you said. There's always a choice. I can't make you come with me. But you're not getting back in your body. And that's just facts. So yes, you can stay. You'll stay here for years. Disembodied, scared, and over the decades it'll probably drive you mad. Maybe you'll even get violent.
Dean: What are you saying?Tessa: Dean. How do you think angry spirits are born? They can't let go and they can't move on. And you're about to become one. The same thing you hunt.
A ghost or spirit remains connected to the mortal realm because it is attached to it through unfinished business or an unwillingness to move on. They may, as Dean nearly did and Bobby did do, refuse to accompany the Reaper who is tasked to help them move on. Many have a desire for revenge against those who they perceive to have caused their deaths, or will target people like them, for example Constance Welch and Peter Sweeney. Others like Dr Ellicott, Dr. H.H. Holmes, and Nurse Glockner were evil in life and continued this after death.
- Molly: I don't understand how a guy like this can turn into that monster.
- Sam: Well, spirits like Greely are like wounded animals. Lost... in so much pain that they lash out.
- Molly: Why? Why are they here?
- Sam: There's some part of them that... that's keeping them here. Like their remains, or unfinished business.
- Molly: Unfinished business?
- Sam: Yeah. It could be revenge. It could be love, or hate. Whatever it is, they just hold on too tight. Can't let go. So they're trapped. Caught in the same loops. Replaying the same tragedies over and over.
- Molly: You sound almost sorry for them.
- Sam: Well, they weren't evil people, you know. A lot of them were good, just something happened to them. Something they couldn't control.
A ghost who can move objects is referred to as a Poltergeist, and when they materialize or possess another person they may exude ectoplasm. Ghosts can also infect a person with Ghost Sickness, a disease that will make the sick person experience the ghost's feelings at the time of their death in slow motion. When Dean is infected, he experiences extreme fear and comes close to suffering cardiac arrest.
Ghosts can affect objects in the material world. This is an ability that must be learned. Sam and Dean learn how to move physical objects and fight when they become astral projections in 4.15 Death Takes a Holiday. The ghost Haskel Crane explains to Bobby and Annie Hawkins that to do this, the spirit must either become very calm and enter a Zen-like state, or tap into "explosive, anger and pure, red-hot rage".
Some ghosts have been shown to possess unique abilities: Freeman Daggett seemed to be able to make Sam disappear and reappear in the blink of an eye, transporting him from the first floor to the basement, Whitman Van Ness could destroy other ghosts and absorb their energy, Mary Worthington could kill anyone through a warped reflection of themselves that accused them of the various deaths they were responsible for. This was also able to be turned back onto her.
Ghosts are not always present by choice or as a result of unfinished business. Ghosts can also be summoned and controlled by witches or demons.
- Walter Dixon uses necromancy to summon the ghosts of past Hollywood actors and crewmen and forces them to kill people who have wronged him.
- The demon Lilith raises the Witnesses, who are spirits of the dead, to attack hunters and break one of The 66 Seals.
- The demon Samhain summons ghosts and raises people from the dead, creating zombies.
- Jimmy Tomorrow binds the ghost of Margaret Fox to him so she will kill other psychics he has a grudge against. 
Over time, the spirits may go mad, or simply degenerate into a state Bobby refers to as "Ghost Alzheimer’s".
The boys commonly force a ghost to move on by salting and burning its corpse. Sam admits that "My dad always said it's like death for spirits. But the truth is, we never know." Bobby Singer later tells Annie Hawkins that he personally believes that ghosts go into oblivion when their bones are burned.. It has been revealed that when a ghost's remains are burned, a Reaper takes their soul to where it belongs, or as in the case of Bobby Singer, somewhere else if an outside source convinces them to.
Burning bones is successful in most cases, including those of Dr Ellicott, Nurse Glockner, Jonah Greely and Whitman Van Ness and his victims. In 5.09 The Real Ghostbusters, the final salting and burning is performed by Demian and Barnes. Sometimes, however, an object can anchor a person's ghost in the living realm. With the Hookman, the Winchesters salt and burn the corpse but also have to melt all the silver from his hook, due to the hook having been part of him in life as well as being the murder weapon. Cyrus Dorian is salted and burnt but only banished when he and his truck are lured onto holy ground. In most cases, the object that must be destroyed to force the ghost to move on contains a small amount of DNA that has been preserved. This is true of Melanie Merchant and Dirk McGregor, whose remaining hair has to be burnt. In the case of P.T. Sandover, his gloves, which may have contained some skin cells or fingernail clippings, have to be burnt. In one memorable case, Rose Brown's kidney, which she donated to her still living sister, Isabel, binds her ghost to a living, moving object. Even though her body is burned by Sam, she cannot be dispatched, and Sam and Dean consider using hoodoo to keep Rose in check. Before they can explore that option, Isabel's sudden death, caused by Rose herself, forces Rose's ghost to move on. In Mary Worthington's case, Sam and Dean were able to turn her power to kill those that have been responsible for other people's deaths with their own reflections back on her by showing her her own reflection.
Iron repels ghosts. For example, Sam uses an iron poker to protect himself and Sarah from Melanie Merchant. The use of iron to repel ghosts has another link with the electromagnetism theory, as striking a ghost with an object made of iron grounds their electric charge, disrupting them and warding them off until they're able to recover.
Sam and Bobby Singer also successfully "scare a ghost to death" by making him relive his torture and violent death, thus effectively dispatching him (Although they only resorted to this questionable method because the circumstances of his death- he was dragged behind a car for a prolonged distance- meant that it was unlikely at best that all of his remains had been collected after his demise as parts of him would have been scattered over the road). This only worked due to the fact that that kind of spirit was created from fear and made of fear so it could be killed with fear. Presumably it wouldn't work for every type of spirit.
Occasionally, a ghost is dispatched when it is forced to deal with its unfinished business. Others must make the choice to cross over, like Molly McNamara, Cole Griffith and Timmy's Mother.
Sometimes one ghost has been shown to be able to destroy another ghost: the ghosts of Constance Welch's children made her disappear when she finally confronted them, Corbett was able to destroy Freeman Daggett and Whitman Van Ness was able to destroy ghosts and absorb their energy into himself.
- Constance Welch, the Woman in White in 1.01 Pilot
- Peter Sweeney, aka the drowned boy in 1.03 Dead in the Water
- Mary Worthington, aka Bloody Mary (urban legend), and Jess in 1.05 Bloody Mary
- Jacob Karns from Hook Man Legend in 1.07 Hookman
- Mary Winchester in 1.09 Home
- Dr Ellicott from Rockford Asylum in 1.10 Asylum
- Cyrus Dorian's spirit possesses his truck in 1.13 Route 666
- Melanie Merchant and Isaiah Merchant in 1.19 Provenance
- Dr. H.H. Holmes in 2.06 No Exit
- Claire Becker in 2.07 The Usual Suspects
- Maggie Thompson in 2.11 Playthings
- Father Thomas Gregory in 2.13 Houses of the Holy
- Molly McNamara and Jonah Greely in 2.16 Roadkill
- Ghosts of old Hollywood actors in 2.18 Hollywood Babylon
- Nurse Glockner in 2.19 Folsom Prison Blues
- Callie in 3.05 Bedtime Stories
- Sailor’s Ghost in 3.06 Red Sky at Morning
- Freeman Daggett, Corbett and the Death Echos in 3.13 Ghostfacers
- The Witnesses in 4.02 Are You There, God? It's Me, Dean Winchester
- Luther Garland in 4.06 Yellow Fever
- Ghost raised by Samhain in 4.07 It's the Great Pumpkin, Sam Winchester
- Dirk McGregor in 4.13 After School Special
- Cole Griffith in 4.15 Death Takes a Holiday
- P.T. Sandover in 4.17 It's a Terrible Life
- Leticia Gore and the Ghost Children in 5.09 The Real Ghostbusters
- Maggie Briggs in 5.12 Swap Meat
- Rose Brown in 6.14 Mannequin 3: The Reckoning
- Jo Harvelle and others controlled by Osiris in 7.04 Defending Your Life
- Kate Fox and Margaret Fox in 7.07 The Mentalists
- Lara Coggins in 7.10 Death's Door
- Bobby in 7.11 Adventures in Babysitting and 7.13 The Slice Girls (unseen)
- Howard's brother in 7.14 Plucky Pennywhistle's Magical Menagerie
- Marin's brother and Bobby (unseen) in 7.17 The Born-Again Identity
- Bobby in 7.18 Party On, Garth
- Bobby, Annie Hawkins, Victoria, Whitman Van Ness, Dexter O'Connell, Haskel Crane and others in 7.19 Of Grave Importance
- Bobby in 7.20 The Girl with the Dungeons and Dragons Tattoo, 7.21 Reading Is Fundamental (unseen), 7.22 There Will Be Blood and 7.23 Survival of the Fittest
- Vance Collins in 8.06 Southern Comfort
- Torvald (off-screen) in 8.13 Everybody Hates Hitler
- Timmy's Mother in 9.07 Bad Boys
- Kevin Tran in 9.14 Captives
Ghosts in Lore
Ghosts or spooks are controversial phenomena which have several possible definitions:
- The spirit or soul of a person who has died, especially one which haunts a place which was of emotional significance to that person when living.
- The personality of a person after his or her own death which is not directly tied to the soul or spirit. A sort of psychic memory-imprint.
- The character or memory of some being or thing which has died or, if it was never alive, been somehow destroyed or disassembled, which nevertheless remains existent (and sometimes detectable) in a semi-corporeal form.
According to reported sightings, ghosts have taken the shape of humans, animals, and even vehicles. There are reports of ghosts (usually of deceased humans) in most cultures throughout the world. There is a great deal of dispute whether ghosts are figments of the imagination or part of objective existence. Ghosts are among the subjects studied by parapsychologists.
- 2.16 Roadkill
- 2.01 In My Time of Dying
- 7.10 Death's Door
- 1.01 Pilot
- 1.03 Dead in the Water
- 1.10 Asylum
- 2.06 No Exit
- 2.19 Folsom Prison Blues
- 1.09 Home
- 4.13 After School Special
- 4.06 Yellow Fever
- 7.19 Of Grave Importance
- 2.18 Hollywood Babylon
- 4.02 Are You There, God? It's Me, Dean Winchester
- 4.07 It's the Great Pumpkin, Sam Winchester
- 7.07 The Mentalists
- 8.19 Taxi Driver
- 1.07 Hookman
- 1.13 Route 666
- 1.19 Provenance
- 4.17 It's a Terrible Life
- 6.14 Mannequin 3: The Reckoning
- 3.06 Red Sky at Morning
- 4.15 Death Takes a Holiday
- 9.07 Bad Boys