The film tells the story of a bullied 12-year-old boy who develops a friendship with a strange child in Blackeberg, a suburb of Stockholm, in the early 1980s.

A film adaptation of Lindqvist's novel began development in 2004 when John Nordling acquired the rights to produce the project.

Alfredson, unconcerned with the horror and vampire conventions, decided to tone down many elements of the novel and focus primarily on the relationship between the two main characters and explore the darker side of humanity.

Alfredson, who had no familiarity with the vampire and horror genres,[11] initially expressed skepticism at having the original author do the adaptation, but found the result very satisfying.

Alfredson felt that the film could not deal with such a serious theme as pedophilia in a satisfying manner, and that this element would detract from the story of the children and their relationship.

[14] Still, the film provided a few hints, of which Alfredson mentions one in the director's comments (Håkan likes children, for the wrong reasons).

[16] The result, which shows Eli slowly beginning to bleed from her eyes, ears, and pores, received positive notices from many critics.

According to an interview with the director, as the film was originally conceived, flashbacks explained this aspect in more detail, but these scenes were eventually cut.

"[13] In the end, Alfredson expressed satisfaction with the result, and has frequently lauded Hedebrant and Leandersson for being "extremely intelligent",[23] "incredibly wise",[26] and "unprecedentedly fantastic.

[20] Its design was intended to suit the CinemaScope format[20] better than a regular jungle gym, which would typically have to be cropped height-wise.

Tracking shots relied on a track-mounted dolly, rather than Steadicam, to create calm, predictable camera movement.

Cinematographer Hoyte van Hoytema and director Alfredson invented a technique they called "spray light".

In an interview, van Hoytema describes it as follows: "If you could capture dull electrical light in a can and spray it like hairspray across Eli’s apartment, it would have the same result as what we created".

[20] The sequence where multiple cats attack Virginia, one of the most complicated scenes to film, required several weeks of drafting and planning.

[31] Footage of Ceylan eating melon or sausage was combined with various animal noises to emulate the sound of Eli biting into her victims and drinking their blood.

Originally an outtake from Gessle's solo album En händig man, the song was specially provided for the film, to resemble the sound of popular 1980s pop group Gyllene Tider.

[36] Other songs in the film include "Försonade" from 1968, written and performed by future ABBA member Agnetha Fältskog,[16] "Flash in the Night" from 1981, written by Tim Norell and Björn Håkansson and performed by Secret Service,[16] and "Dags å välja sida" by Peps Blodsband.

[42] The Swedish premiere was originally planned for 18 April 2008, but following the positive response from the festival screenings, the producers decided to postpone the release until autumn, to allow for a longer theatrical run.

[43] At one time there was a plan to release the film for a special series of screenings in Luleå, beginning 24 September and lasting seven days.

[48] Following customer complaints, Magnet stated that they would release an updated version with the original theatrical subtitles, but will not exchange current discs.

The critical consensus reads, "Let the Right One In reinvigorates the seemingly tired vampire genre by effectively mixing scares with intelligent storytelling".

[54] Svenska Dagbladet gave the film a rating of five out of six, and hailed Alfredson for his ability to "tell [stories] through pictures instead of words about a society where hearts are turned to icicles and everyone is left on their own, but also about love warm and red like blood on white melting snow".

Expressen criticised it for being unappealing to those uninitiated in vampire films while Göteborgs-Posten believed the supporting characters had lost the emotional depth that made the novel so successful.

[59] KJ Doughton of Film Threat thought the visuals in the ending were fresh and inventive and would be talked about for years to come.

[53] One negative review came from Owen Gleiberman of Entertainment Weekly, who gave the movie a "C", characterizing it as a "Swedish head-scratcher", with "a few creepy images but very little holding them together".

In their rationale, the authors noted that, "in these days where every second movie seems to feature vampires, it takes a very special twist on the legend to surprise us – but this one knocked us out and then bit us in the jugular", and found that the "strange central friendship" between the two lead characters was what made the film "so frightening, and so magnetic".

[38] Let the Right One In was nominated in five categories for the Swedish Film Institute's 2008 Guldbagge Award, eventually winning for best directing, screenplay and cinematography as well as a Best Achievement-award to production designer Eva Norén.

[69] When the Swedish Film Institute on 16 September announced that Jan Troell's Everlasting Moments had been selected instead of Let the Right One In, the Luleå screenings were canceled.

[20][103][104] Alfredson was initially asked to helm the remake, but he turned it down stating that "I am too old to make the same film twice and I have other stories that I want to tell.

[106] Let Me In was released in late 2010 starring Chloë Grace Moretz and Kodi Smit-McPhee as Abby and Owen, Eli's and Oskar's respective counterparts, and received very positive reviews but underperformed at the box office.