Killstreak

A killstreak is earned when a player acquires two or more kills in a row without dying.

In the multiplayer of Call of Duty 4: Modern Warfare, Call of Duty: World at War, Call of Duty: Modern Warfare 2 and Call of Duty: Black Ops, the player is given various rewards depending on how long the killstreak is. Most killstreaks provide air support, but some, like the UAV and the EMP, can help the user's entire team.

Call of Duty 4: Modern Warfare

 * 3 - Radar: Deploys a UAV to scan the map for 30 seconds and reveal enemy locations (enemies with the second tier perk UAV Jammer will not be scanned by the UAV).
 * 5 - Airstrike: Initiates an airstrike on a targeted area. Note: Enemies cannot be spotted on the map unless a UAV is active, or they are shooting, so that decreases an air strike's accuracy.
 * 7 - Helicopter: Deploys either an Mi-24 hind or an AH-1 Cobra, depending on what team the player is on, to attack enemy players for 1 minute. Note: It can be shot down with RPGs, launched grenades, or concentrated fire on the area just below the rotors.

Call of Duty: World at War

 * 3 - Recon Plane: Same effect as UAV Recon.
 * 5 - Artillery: Calls in eight artillery shells to strike a designated location. Player will experience shellshock if close to where the artillery is landing.
 * 7 - Dogs: Releases several vicious dogs for one minute that attacks enemy players. If the dogs are the player's team they will be German Shepherds. All enemy dogs show up as black dogs, possibly Rottweilers.

Call of Duty: Modern Warfare 2
In Modern Warfare 2, players can choose up to three killstreak rewards they can earn, rather than just the standard three. At the start, the player can only use UAV, Care Package and Predator Missile, with the option to unlock one new killstreak at each of the following levels: 10, 15, 23, 35, 47, 51, 55, 59, 63, 66, 68, and 69 (all killstreak rewards will be unlocked at level 69). When a killstreak of 8 or more is activated, a message will notify all players in the match, showing the callsign and the streak being used (except in Hardcore or when the killstreak is a Stealth Bomber being deployed by the opposing team). Even though a Tactical Nuke is announced to both teams, the player's callsign on the top-right corner will only be shown to players on that player's team.

It is worth noting that two killstreaks that share the same killstreak amount cannot be used together, one must be picked over the other. For example, for the 7 kills slot the player can use either the Harrier Strike or the Attack Helicopter, but not both, As well with 9 kills slot the player can use either the Pave Low or the Stealth Bomber and the 11 kills slot the player can choose from either the AC130 or the Chopper Gunner.

For controllable killstreaks (Predator Missile, Chopper Gunner and AC130), if the player is killed while operating the killstreak, the player can continue using the killstreak until it is done and then respawn or enter spectator mode as normal. The only exception to this is Search and Destroy. This outcome has been protested by some who feel the killstreak should end if the player dies. Note: The number of kills required to obtain a particular killstreak reward can be reduced by one (1) when using the Hardline perk.

Another note: there are some glitches for a certain few of the Killstreaks concerning Cold-Blooded concerning the player getting all his/her Cold-Blooded Pro Challenges.

Call of Duty Black Ops
Black Ops will feature the same customizable killstreak reward system as Modern Warfare 2. All killstreak rewards will be available to purchase, via CoD Points, at rank 10. Kills from killstreak rewards will no longer count towards higher killstreaks unlike previous titles, which means player must kill the required number of enemies themselves. Also, the Tactical Nuke or other game-ending killstreak rewards will not appear.

Trivia

 * In Call of Duty 4: Modern Warfare, and Call of Duty: World at War, killstreaks do not stack, meaning that once a killstreak reward is available it replaces any unused killstreak reward. In Call of Duty: Modern Warfare 2 they stack with the most recent rewards earned being first in the queue.
 * Killstreaks in Call of Duty: Modern Warfare 2 that count towards other killstreaks will only do so during the same life. For example, a Predator Missile can contribute towards any other killstreak the player has selected of 6 or higher, but if used after the player dies, it will not count towards any killstreaks. Killstreaks from Care Packages and Emergency Airdrops don't count towards the player's current streak.
 * In Call of Duty: World at War, there originally were Fourth Killstreak and Carpet Bomber killstreaks.
 * In Call of Duty: Modern Warfare 2, killstreaks were originally divided into categories and players could select up to three of them.
 * In Call of Duty: Modern Warfare 2, the UAV, Counter UAV and EMP are the only killstreaks that do not directly kill enemy players.
 * In Call of Duty: Modern Warfare 2 there are chatter sounds for Little Bird and Airdrop (not Care Package or Emergency Airdrop) killstreaks. However, these killstreaks are cut from final game. Also, chatter sounds for enemy Predator Missile, Sentry Gun, and Airstrike can be found in game files, but they're not used.
 * In Call of Duty: Modern Warfare 2, if one switches teams on a private match after having earned a killstreak without using the killstreak first, the player will lose the killstreak.
 * Killstreaks are still destructible after a match ends, meaning if the player fires a rocket or AC130 guns at an enemy aircraft, one can still get points if the match ends before the rocket/round hits.
 * In Modern Warfare 2 when someone deploys a non-controllable killstreaks, if you change teams the killstreaks will change to red or green depending on the team and will still attack its own teammates.