XP

Experience Points (XP) are a numerical quantity that dictates your level and your progress in that level. Your experience points are shown (in multiplayer) on the bottom of the screen with a yellow meter. When you gain experience points, it becomes green and resets when you achieve the next level.

The Experience Meter is only available in Call of Duty 4: Modern Warfare, Call of Duty: World at War, and Call of Duty: Modern Warfare 2. Experience points are also available in Call of Duty: United Offensive but there is no experience meter and the ranks reset when you end or exit a match.

Call of Duty: United Offensive
You only have to kill enemy players; destroy bases in the Base Assault gamemode or capture flags. You start as a private-first class and the top rank is Sergeant. The ranks allows you to carry more grenades, satchel charge and a binocular for artillery strike.

Call of Duty 4: Modern Warfare

 * Killing the enemy. You gain 10 points per kill in Team Deathmatch (all forms) and Sabotage. 5 points are earned for killing in Free for All, Headquarters and Domination. You can gain 50 points per kill in Search and Destroy, as no respawns are permitted; the life is worth more. If you kill when using Last Stand perk, double points will be earned. In Headquarters or Domination, you score an extra 5 experience points for kill an enemy who is within an objective capture range (i.e. within a Domination flag).


 * Assisting in the death of an enemy. If you attack an enemy but do not kill him and he dies before his health is fully regenerated you are awarded an assist, 2 experience points for matches which grant 10 exp a kill, 1 for kills which grant 5 exp a kill, and assists are impossible to achieve in Free-for-all mode.


 * Completing challenges. Challenges are a new method to gain experience points. Challenges have some requirements to be fulfilled before the reward is available. For example, for marksman challenges, you need to kill the enemy using the pertinent gun or in expert challenges, you need kill the enemy via headshots with the gun. There are also challenges that require you to do other, more varied tasks, for example, the "think fast" challenge requires you to kill an enemy by directly hitting the enemy with a Fragmentation Grenade.The challenges involving guns gives either attachments or camouflages as an addition to experience points. Other challenges give only experience points.


 * Additionally: The game gives you some bonus points for your performance when the match ends as a "Match Bonus".

Call of Duty: World at War
You can gain experience points in the same manner as you do so in Call of Duty 4: Modern Warfare, but also by:


 * Cooperative (Co-op) gameplay. This is a new mode where you play the missions in normal gameplay but with 2-4 players. You can do exclusive challenges when playing in Co-op. Playing Nacht der Untoten does not gives any experience points. Playing in split screen cooperative mode also does not give any experience points


 * Completing vehicle challenges


 * Reviving a teammate whom is using Second Chance. This gives you 2 points.


 * Gaining an assist. This is more complicated than Call of Duty 4: Modern Warfare as the amount of experience points gained is proportionate to the amount of damage inflicted towards the enemy. Normally +20 or +40 exp.

Call of Duty: Modern Warfare 2
In this game, the XP system was completely redone, given points are increased ten-fold in comparison to MW1. There are many XP bonuses for doing special things, including:


 * Payback! - Kill an enemy who has killed you previously. +50XP Getting this for a particular opponent will reset it until they kill you again.
 * Buzzkill! - Kill an enemy who is 1 kill short of acquiring kill streak reward. +100XP
 * Bulls-eye! - Kill an enemy with a throwing knife which scores a headshot. +50XP
 * Flag Runner! - Picked up the flag in a Capture the Flag match. +50XP
 * Stuck! - Stick a Semtex grenade on an enemy. +50XP
 * Rescuer! - Save a wounded teammate +50XP Kill the person attacking him
 * Hijacker! - Steal enemy care package.+50XP Sometimes more
 * First Blood! - Be the first person to score a kill in a match. +150XP
 * Offense! - You killed a defender. +50XP
 * Comeback! - You've recovered from a streak of deaths. +50XP More dependant on amount of deaths in death-streak
 * Position Secure! - You've taken an objective. +150XP
 * Double Kill! - Kill two enemies at once. +50XP
 * Triple Kill! - Kill three enemies at once. +75XP
 * Multi Kill! - Kill four or more enemies at once. +100XP
 * Longshot! - Kill an enemy from a distance. +50XP
 * Share Package! - A team-mate saved your Care Package from the enemy! Multiples of +50XP dependant on award given. (I.E. Friendlies rescuing an AC-130 will score more for you than if they rescued a UAV)
 * One Shot Kill - Kill an enemy with one shot.* +100XP Does not award XP, except for maybe the first times. 
 * Flag Return - You return the flag to it's "base". +50XP
 * Headshot - You kill the enemy with a bullet to the head. +50XP
 * Crowd Control - Make an excellent distraction (using the riot shield). Multiples of +10XP dependant on damage absorbed.
 * Afterlife - Kill an enemy while dead. +25XP
 * Assisted Suicide - Injure an enemy before he kills himself (usually due to fall damage). +350XP
 * Execution - Headshot someone who is in Last Stand or Final Stand.+100XP
 * Avenger - Kill an enemy who has killed your team-mate.+50XP
 * Never Forget! Get hurt by an enemy then knife him in the back +100XP


 * This does not apply to Shotguns as the shells they fire spread into several smaller 'pellets' which means it's actually taking you several shots to score that kill even though you may have only pulled the trigger once.