Dog Tags

Dog tags are two flat metal tokens attached to a small chain that are carried by military personnel.

Call of Duty 4: Modern Warfare
Dog tags can be seen in one instance in Call of Duty 4: Modern Warfare, being carried by the dead soldier Paulsen. They are retrieved by Nikolai. A dog tag can also be seen in Call of Duty 4: Modern Warfare hanging from the combat boots of Marine Force Recon soldiers. This is maybe to help identify the corpse in the event that the dog tag round his neck is blown off.

Call of Duty: World at War
In Call of Duty: World at War, in the campaign mission "'Breaking Point'", depending on the player's actions, Sgt. Roebuck will give Polonsky's dog tags to Miller. If the player saves Polonsky instead, then Polonsky gives Roebuck's dog tags to Miller. Roebuck/Polonsky was killed minutes before by suicide grenadiers. In the mission "Their Land, Their Blood", in the opening sequence the player can see a German soldier tossing around a dog tag. Also, on the death cards hanging from the rifles will be dogtags with the Treyarch symbol on it.

Call of Duty: Modern Warfare 2
Dog tags are seen once in Call of Duty: Modern Warfare 2, appearing as an emblem in Multiplayer. They are the player's starting emblem, and can be on a player's Callsign at any time.

Call of Duty: Modern Warfare 3
In MW3, dog tags are seen in a new type of game mode called Kill Confirmed. After the player has killed an enemy, he/she has to collect dog tags that the enemy drops to get points for the team. If another enemy takes the dog tags the kill will be taken away. No matter if the player does collect the dog tags or not the kill will count to the player's personal xp.

Trivia

 * During online gameplay, when the player is killed, a dog tag will appear beside the killed player's name in the leaderboards, representing his/her death.