Call of Duty is a First Person Shooter that takes place during Worl War II. It was released Oct 29, 2003 for the PC, published by Activision and developed by Infinity Ward.

The game is based on the Queake III: Team Arena engine. It was accompanied in September 2004 by an expansion pack, Call of Duty: United Offensive, which was produced by Activision, and developed by Gray Matter Interactive, with contributions from Pi Studios. Call of Duty is similar in theme and gameplay to Medal of Honor, as it is made out of single-player campaigns and missions. However, unlike Medal of Honor, the war is seen not just from the viewpoint of an American soldier but also from the viewpoint of British and Soviet soldiers. Also unlike many other war games Call of Duty introduced more computer-controlled allies who vary in quantity, providing the player with support throughout the games offering a more realistic WWII experience. Call of Duty also featured "shellshock" (not to be confused with the psychological condition of the same name): when there is an explosion near the player, he momentarily experiences simulated tinnitus, appropriate sound "muffling" effects, blurred vision, and the action moves slower.

A port of the game, known as Call of Duty Clasic, has released on the Playstation Network and Xbox Live Marketplace available to be downloaded since November 10th 2009. If gamers wish to buy or have bought the Hardened or Prestige editions of Call of Duty: Modern Warfare 2, it will contain a token they can use to download the game for free. This copy of the game is an exact copy, no sprinting, no perks, no ranks.

There are many mods for Call of Duty. A few of the popular mods include:

• Revolt: Changes the Allies And Axis into a 21st century coalition versus resistance style of gameplay. The normal World War II era weapons are changed to look and work like modern weapons.
• Heat of Battle: Commonly known as "HOB", Heat of Battle offers several new gametypes and maps. There are also new features such as minefields which protect spawn points. A capture-and-hold style gametype was also added.
• SWAT: Changes the Axis and Allies into a modern day face off between criminals and the SWAT forces. The weapons are replaced with modern weaponry, and new game types are added. In one particular game type, one player on the SWAT team is designated as the "Target" or "Hostage" and must be escorted by SWAT forces to the extraction point while criminal forces try to kill the "Target".
• AWE (Additional War Effects): Commonly referred to as "A-W-E", AWE adds features to the game such as cooking grenades, portable MG42s, bleeding, and sprinting, maps, tanks, halftracks, field guns, mortars, and other features. AWE also fixes some commonly exploited bugs in the maps and scripts of Call of Duty. AWE is a server-side mod.
• Breaking 50 Paintball: Usually called the "ballin'" mod due to "pb" being PunkBuster. In this alteration of the game each side may pick from paintball guns. Along with the guns they are given paint grenades.