Martin Scorsese included this movie on his Top 11 Scariest Horror Films of All Time list.

The real-life Carla Moran's teenage son described a particularly vicious attack in which Carla was thrown by the malevolent force and hit her head. He tried to intervene, but he was also thrown, breaking his arm. In the filming of the movie, the actor playing the son broke his arm in that scene, and the curtains tore from top to bottom without explanation.

The paranormal and supernatural events depicted in this movie purportedly first took place in 1974, which was around seven to eight years before this film was made and released.

Composer Charles Bernstein's score for this film is excerpted in Quentin Tarantino's Inglourious Basterds (2009).

The method in which Carla is diagnosed by physicians, a method which relies heavily on her personal history, and in particular her relationship with her father and her sexual encounters with other men, reflects a largely Freudian psychodynamic method of diagnosis. This, combined with the fact that she is initially diagnosed by the team of doctors as having "hysteria," a disorder that has disappeared from mainstream American psychiatric diagnostics, makes this movie one of the last in which Freudian methods and conceptions (largely related to sex and childhood development) are shown to have a significant impact on the diagnosis of patients. With the rise of neurochemistry, neuroscience, and biological tests in the 1980s, 1990s and beyond, Freudian methods like those portrayed in the film would themselves become considered by many mainstream biological psychiatrists to be superstitious.

Actresses Jane Fonda, Jill Clayburgh, Bette Midler and Sally Field were initially sought for the role played by Barbara Hershey.

In a July 2012 interview published in Rue Morgue magazine, the film's director, Sidney J. Furie, said that he did not consider this movie to be a horror film.

For the scenes where the entity assaults Carla Moran, the visual effects team designed a prosthetic breastplate and full body that could be "indented" from below... pretending fingers that "touch" her body.

Both George Coe (Dr. Weber) and Alex Rocco (Jerry Anderson) died on July 18, 2015.

This movie was originally planned to be released in 1981 but did not debut in theaters until late 1982, with some territories including Australian and its American release in the U.S. not launching until early 1983. Generally, the film was released a short time after 1982's other poltergeist movie, Poltergeist (1982).

A whole dream sequence where Carla was forced to have incestuous thoughts about her son by the Entity was dropped for the movie by director Sidney J. Furie, because it was too sexually controversial at that time. This was despite the then recently released Bernardo Bertolucci film, Luna (1979), which examined a mother-son relationship and also being from the same 20th Century Fox studio.

The Bollywood film Hawa (2003) was based on this film.

This film was made and released about four years after its source novel of the same name by Frank De Felitta was first published in 1978. A frequent dust-jacket blurb for the book read, "Beyond physical reality, beyond ecstasy and pain, to a dark netherworld of psycho-sexual truth."

This movie is based on the real life attack of a Californian woman named Doris Bither. According to Bither, she was constantly raped by the spirits of three men. Two would hold her down while the third raped her.

This was one of two supernatural horror movies made during the late 1970s and early 1980s that were adapted from a novel by Frank De Felitta. The other was Audrey Rose (1977).

Robert MacNaughton auditioned for a role for this film after being asked by the casting director, who saw him in an off-Broadway play in New York.

In this movie, Barbara Hershey's character is pursued by an evil being. Eventually, she played a character who is the mother of a family man pursued by an evil being, in Insidious (2010) and Insidious: Chapter 2 (2013).

The film's closing epilogue states, "The film you have just seen is a fictionalized account of a true incident which took place in Los Angeles, California, in October 1976. It is considered by psychic researchers to be one of the most extraordinary cases in the history of parapsychology. The real Carla Moran is today living in Texas with her children. The attacks, though decreased in both frequency and intensity...continue." According to an update by the Conneticut Paranormal Research Society web-page article titled "The Carla Moran Story, the Entity,""The woman [Carla Moran] moved five times, but the attacking entity followed her. She eventually moved further away. As she moved, the phenomena diminished, and after about two years, the attacks stopped altogether."