Shot in Romania, most of it was filmed in Nicolae Ceausescu's unfinished Academy of Sciences building in Bucharest.
The film was originally scripted as a straightforward remake of the 2003 South Korean horror film Into the Mirror (2003). However, once Alexandre Aja was brought on board and read the script, he was dissatisfied with the particulars of the original film's story. He decided to retain the original film's basic idea involving mirrors, and to incorporate a few of its scenes, but change the story dramatically.
Since this was the feature film debut of Cameron Boyce, and he was only 9-10 during production, his dad said that he wasn't allowed to watch the film at the time.
Grégory Levasseur had to fill in as director for the scene where Kiefer Sutherland thinks he's on fire as Alexandre Aja had to leave urgently. His wife had just gone into premature labor.
In the bar scene, make-up artist Greg Nicotero can be seen as an extra playing pool. He was also in the make-up department for the film.