Advent Children began as a short film concept by Visual Works, the company Square used to create CGI cutscenes for its video games, and was originally planned to be only about 20 minutes long.

Scriptwriter Kazushige Nojima decided the story would focus on Cloud and Tifa and the children, and the original short script involved a message being relayed to Cloud through several children before the messenger's identity was revealed.

The project grew from a planned 20-minute short to a 100-minute feature after early word of the film generated huge interest among Final Fantasy VII fans who wanted something feature-length.

Early in pre-production the team considered turning Advent Children into a video game, but director Tetsuya Nomura decided against it, partly because Visual Works had no experience developing a full game.

Directing duties were split between Tetsuya Nomura and Takeshi Nozue, who had previously worked together on Kingdom Hearts; Nomura felt sharing the role would add depth to the film.

Motion capture was used extensively for the film's battle scenes, allowing the animators to create maneuvers that would have been physically impossible for live actors to perform.

The battle between Cloud's group and Bahamut SIN was the most difficult sequence to design because of the size of the battle area and the huge number of objects the staff had to add to keep the scene realistic.

Character designs had to be reworked from their original game appearances; Cloud's final design was a combination of eight different concepts ranging from his super-deformed video game look to a more realistic cinematic appearance.

Designing Sephiroth proved so difficult that the staff reduced his number of appearances in the film, and it took them two years to develop and refine his final look.

Kadaj, Loz, and Yazoo were each designed to embody a different facet of Sephiroth's spirit—cruelty, strength, and allure respectively—and were made younger than Cloud to emphasize a 'next generation' theme.

The character Loz was modeled on a staff member's grandson according to the audio commentary; the child's real phrases like 'let's play' and an onomatopoeic word were incorporated into Loz's dialogue.

Advent Children was invited to the Venice International Film Festival in 2004 while still unfinished, and returned the following year as a completed film, an unusual case of two consecutive festival appearances.

The film became one of the best-selling animated releases in Japan, selling out quickly on release and simultaneously placing three different editions in the top five of Japan's Oricon weekly DVD chart, with the standard edition eventually returning to number one in its fifth week.

By 2009 the film had shipped over 4.1 million copies worldwide, an unusually high figure for a direct-to-video release.

A Korean singer's (IVY) music video was found by a Seoul court in 2007 to have illegally copied characters, backgrounds, and story elements from Advent Children, resulting in criminal fines and a damages award of 300 million won against the label and director.

The film's ending theme 'Calling' was originally released by rock singer Kyosuke Himuro in 1989, but producer Tetsuya Nomura, a fan of Himuro, kept the 15-year-old song in mind throughout production and it was used in the finished film.

For the Complete edition, Himuro recorded a new ending theme titled 'Safe and Sound' in collaboration with My Chemical Romance singer Gerard Way.

Panasonic produced a real Japan-exclusive FOMA P900iV cell phone identical to the one Cloud uses in the film, complete with wallpapers and ringtones tied to Advent Children.

Yazoo's weapon, a blade-gun hybrid called 'Velvet Nightmare,' is designed to resemble a miniaturized version of the gunblade used by Squall, the protagonist of Final Fantasy VIII.