The Exorcist is an influential and successful 1973 horror film, adapted by William Peter Blatty from his 1971 novel of the same name.
Directed by William Friedkin and starring Max von Sydow as Father Lankaster Merrin, Ellen Burstyn as Chris MacNeil, Jason Miller as Father Damien Karras, Lee J. Cobb as Lieutenant William Kinderman and Linda Blair as Regan MacNeil. Regan's voice was dubbed by Mercedes McCambridge when possessed.
Blatty based his novel on a supposedly genuine exorcism from 1949, in Cottage City, Maryland.  Several area newspapers reported on a speech a minister gave to an amateur parapsychology society, in which he claimed to have exorcised a demon from a thirteen-year-old boy named Robbie, and that the ordeal lasted a little more than six weeks.
In the film, a young girl named Regan, living in Georgetown, Washington, D.C., with her mother, (a famous actress) becomes ill after playing with a ouija board. She undergoes a series of physical and psychological changes.
After unsuccessful medical help, Regan's mother turns to religion. The girl is examined by a priest, Father Damien Karras, who is convinced of the diabolical nature of the case. He turns to the local bishop, who appoints a second priest, Father Merrin, to perform an exorcism. The lengthy exorcism tests the priests, both physically and spiritually.
The Exorcist contained a number of disturbing special effects, engineered by makeup legend and pioneer Dick Smith . The effects were so graphic that Roger Ebert writes "That it received an R rating and not the X is stupefying."
The Exorcist was also accused of, among many other things, manipulation of its audience through the use of subliminal imagery; a claim that is verifiably false upon a viewing of the film. The imagery in question is readily apparent and easily recognizable as a simple, yet effective editing technique, designed to make the viewer ill at ease - the desired effect for a horror film, after all.
The film originally contained several key sequences from the novel, which were cut prior to release by director Friedkin, despite Blatty's protests. These scenes were later restored and--along with a number of new digital effects--inserted into the re-release subtitled "the version you've never seen" in 2000.
The film was a huge international hit, grossing as of 2004 $402,500,000 worldwide. It was nominated for ten Academy Awards (winning two), and also won four Golden Globes. McCambride's role was originally uncredited; after Blair was nominated for her role, McCambridge initiated a lawsuit seeking redress. (Incidentally, McCambridge's voice was not processed or treated for The Exorcist, she had worked extensively in radio drama and had a flexible vocal range.)
The Exorcist is commonly regarded as one of the best and most effective horror films; one that balances a stellar script, gruesome effects, and outstanding performances.
Interestingly, the part of Regan was originally offered to troubled actress Dana Plato, whose mother refused to allow her to take it.
John Boorman's poorly-received Exorcist II: The Heretic was released in 1977.
1990's more successful The Exorcist III, written and directed by Blatty himself from his own 1983 novel Legion, the true sequel to the original novel. Exorcist III ignored the events of the previous sequel and presented a satisfying conclusion to the story after 15 years.
A parody entitled Repossessed was released the same year, with Blair lampooning the role that made her (in)famous.
A prequel, Exorcist: The Beginning (2004) attracted controversy even before its release. The original director Paul Schrader was fired after the studio did not like his film (working title The Exorcist: Dominion). The whole film was then re-shot with Renny Harlin directing, and the original film shelved. Harlin's version was poorly received.
However, the film's producers are considering releasing the Schrader-directed film as a separate direct-to-video release.
There have been rumors that the various Exorcist films were cursed.. Blatty, Schrader and von Sydow have discounted such tales as nonsense, used primarily for promotion.