Killstreak

A killstreak is when a player gets two or more kills in a row without dying. In Call of Duty 4: Modern Warfare, Call of Duty: World at War, and Call of Duty: Modern Warfare 2 the player is given various rewards depending on the length of the killstreak.

Call of Duty 4: Modern Warfare

 * 3 - UAV: Deploys a UAV to scan the map for 30 seconds and reveal enemy locations.
 * 5 - Airstrike: Initiates an airstrike on a targeted area
 * 7 - Attack Helicopter: Deploys an armored helicopter to attack enemy players.

Call of Duty: World at War

 * Recon Plane - 3 kills
 * Artillery Strike - 5 kills
 * Dogs - 7 kills

Call of Duty: Modern Warfare 2
In this game, 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, 56, 59, 63, 66, 68, and 69. When a killstreak of 8 or more is activated, a message will notify all players in the room, showing the callsign and the streak being used (except in Hardcore or when the killstreak is a Stealth Bomber). 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 you can only use either the Harrier Strike or the Attack Helicopter.

The killstreaks available are:


 * 3 - UAV: Deploys a UAV to scan the map and reveal enemy locations. Now able to be shot down.
 * 4 - Care Package: "Air drop a random killstreak or ammo". Deploys by throwing a red smoke grenade obtained after getting a 4 killstreak. The care package can be picked up by anyone once it hits the ground, including the enemy team. The Care Package can also kill if it lands on a player. Kills via care package rewards do not count toward the killstreak, but kills from a crate landing on an enemy do.
 * 4 - Counter-UAV: Deploys a UAV that disables the enemy's radar, and renders hostile UAVs useless.
 * 5 - Sentry Gun: Allows the player to deploy an automated M5 Minigun Turret. The gun automatically fires at enemies in its line of sight, but it can be destroyed by enemy fire. Once deployed, the player who placed it can pick it back up and redeploy it somewhere else. However, moving a sentry gun results in further sentry kills not counting toward a killstreak.
 * 5 - Predator Missile: Fires one controllable missile at a designated target area. User is vulnerable during use.
 * 6 - Precision Airstrike: Initiates an airstrike on a targeted area much like the Airstrike in Call of Duty 4. However, the user can now control the direction the airstrike comes from.
 * 7 - Harrier Strike: Deploys a small airstrike followed by a hovering harrier that will attack enemy players for a short time.
 * 7 - Attack Helicopter: Deploys a Hind (for Spetznaz, Militia and OpFor teams) or a Cobra armed with one minigun to attack enemy players.
 * 8 - Emergency Airdrop: Deploys a Cargo plane that drops four care packages.
 * 9 - Pave Low: Deploys a Sikorsky MH-53 armed with two gun turrets. Currently bugged: pave low kills do not count toward killstreak. However, obtaining another kill before the pave low gets any will fix this glitch temporarily.
 * 9 - Stealth Bomber: Deploys a B2 Spirit that is invisible on the enemies radar. The bomber drops a large carpet bomb.
 * 11 - Chopper Gunner: Deploys a AH-64 Apache attack helicopter that the player controls. User is vulnerable during use.
 * 11 - AC-130: Deploys an AC-130 gunship that the player controls. User is vulnerable during use.
 * 15 - EMP: Deploys an Electromagnetic pulse that destroys enemy killsteaks and disables the enemies' HUD, radar and any other device that uses electronics (various attachments, C4, Claymores, Semtex, etc.)
 * 25 - Tactical Nuke: Calls in a nuclear weapon that detonates after 10 seconds, killing every player in the match and ending it in favor of the bomb user's team, even when losing.

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.
 * It is debatable whether or not kills from specific killstreaks count towards lower killstreaks, should the player die before the killstreak runs its course. This was a common strategy before Modern Warfare 2, where the highest kill streak reward (7 kills) would best be used immediately after respawning to re-earn the 3 and 5 kill streaks.  However, in Modern Warfare 2, killstreaks do not count towards lower killstreaks (i.e. 3 kills with a Harrier after respawning will not unlock a UAV).