Thief: The Dark Project is a 1998 first-person stealth video game and also an earlier example of the immersive sim genre developed by Looking Glass Studios and published by Eidos Interactive.

Set in a fantasy metropolis called the City, players take on the role of Garrett, a master thief trained by a secret society who, while carrying out a series of robberies, becomes embroiled in a complex plot that ultimately sees him attempting to prevent a great power from unleashing chaos on the world.

To assist them in remaining hidden, a special meter on the heads-up display (HUD), in the form of a gem, helps to indicate the player's visibility to NPCs; the brighter it is, the more easily they can be visually detected, thus sticking to dark, shady spots where the gem dims ensures the player is hidden, though NPCs can still find them if they get too close in front of them.

[8] If an NPC sees or hears something out of place, they will react to it, depending on the level of its suspicions; if for a brief second, they will simply ignore it, but if for long enough, they will become alert to their surroundings and begin searching the area.

[6] Non-human characters range from giant spiders and feral creatures to zombies and ghosts, with certain levels containing survival horror elements.

[3][12] Thief takes place in a metropolis called "the City",[6] which has been noted to contain elements of the Middle Ages-like dark fantasy and the Industrial Revolution.

[4] Warren Spector, who had recently left Origin Systems to found Looking Glass Studios Austin, became Dark Camelot's producer after his predecessor departed.

[59] Levine said inspiration for the idea of being powerful when undetected but very vulnerable when exposed came from the concept of submarine warfare and in particular from the 1985 simulation video game Silent Service.

[52] Multiplayer support was planned, including the theftmatch mode where small teams of thieves compete under time pressure to steal the greatest value of swag from the territory of wealthy NPC's and their guard.

The company's Austin branch closed, costing Spector and several game engine programmers; this team relocated to Ion Storm, and released Deus Ex in 2000.

[52] By April 18,[59] Looking Glass Studios laid off half of its entire staff in six months, which damaged morale of The Dark Project team, which at this point was vastly different from the one with which the development began.

"Few emotions can compare to the stress of heading to work not knowing who might be laid off, including yourself, or whether the doors would be locked when you got there", lead programmer Tom Leonard later said.

This stress caused several team members to voluntarily quit, including the lead programmer, Briscoe Rogers,[59] who had designed the game's AI system, which suffered from software bugs and problems with complexity.

Further, the release of games like Commandos: Behind Enemy Lines, Half-Life and Metal Gear Solid eased worries that experimental gameplay styles were unmarketable.

Leonard later demonstrated that first-person shooters, like Half-Life, often utilize "look and listen" AI systems, wherein NPCs become aggressive when the player is seen or heard.

[10] Certain levels included horror elements,[11] and one such mission, Return to the Cathedral, intentionally removes players' ability to judge their vulnerability.

Kieron Gillen of PC Gamer UK believed that the level creates "a cycle of relaxation and abhorrence [... that results] in a devastating pummelling of the nerve endings".

[4] The team extended the concept by decreasing the player's ability to kill human characters on higher difficulty settings.

[62] Project director Greg LoPiccolo wanted Thief's audio to both enrich the environment and enhance gameplay, and the game's design necessitated an advanced sound system.

[67] Thief: The Dark Project received critical acclaim[69] from publications including The Washington Post,[78] PC Gamer,[75] and Salon.com.

With directional noises and haunting 'background' effects you are plunged into Garrett's shadowy world and left with a pounding heart and twitchy nerves.

"[71] Wagner James Au of Salon.com noted that the game's level of suspense was "exquisite" and that its use of detailed aural cues as a gameplay device bordered on virtual reality.

[83] Its global sales reached 500,000 copies by May 2000, making it Looking Glass Studios' most commercially successful game, according to the Boston Globe.

[86] The game has been cited as the first to use light and shadow as a stealth mechanic,[13] and the first to use audio cues, such as the ability to eavesdrop on conversations and alert guards with loud footsteps.

"[13] Laidlaw called Thief his favorite game,[88] an opinion shared by Fallout 3 lead designer Emil Pagliarulo,[89] and Michel Sabbagh of Bethesda Softworks.

Inducting it into its hall of fame, GameSpy writer Rich Carlson wrote: "With a tactical philosophy contrary to nearly every [first-person shooter] action game at that time, Thief rewarded stealth and sneaking over brazen frontal assault", continuing: "While inadvertently undermining the notion that all action games need be shooters, it carved a completely new niche in the same already glutted genre.

[87] John Walker of Eurogamer wrote in a retrospective review: "Thief is an embarrassment to modern stealth games, each of which produces only a faded parody of this masterful original.

Thief: Deadly Shadows, released for both Windows and the Xbox, was developed by Ion Storm due to the 2000 closure of Looking Glass Studios.

In May 2009, a fourth game, simply titled Thief was revealed to be in development by Eidos Montréal for Windows, PlayStation 3 and Xbox 360.

Standalone fan made remake The Dark Mod aims to recreate the "essence" of Thief in a modern game engine.