Perks

Perks (also known as Specialties) are modifiers in multiplayer only in Call of Duty 4: Modern Warfare, Call of Duty: World at War, Call of Duty: Modern Warfare 2, Call of Duty: Black Ops and Call of Duty: Black Ops (Nintendo DS) that passively affect game play, allowing further customization of classes. They affect available equipment, effectiveness of the player's equipment, the usefulness of the enemy's equipment, and the physical properties of the player's character.

Call of Duty 4: Modern Warfare Perks
Some perks are available initially while others are unlocked as the player progresses through multiplayer ranks. Up to three perks, one of each tier, can be equipped at one time. The tiers are color coded blue, red, and green and therefore only one of each can be equipped.

First Tier
These perks deal with a player's equipment and/or explosives. First tier perks are colored blue.

Attachments
Tier 1 perks can also be replaced by certain attachments.
 * Grenade Launcher - an attachment for Assault Rifles.
 * Grip - an attachment for Shotguns and LMGs.

Third Tier
These perks give the player various abilities. Third tier perks are colored green.

Call of Duty: World at War Perks
Like in Call of Duty 4: Modern Warfare, the perks are divided into equipment, weapon and ability tiers.

Attachments
These attachments remove Perk 1.

Rifle Grenade Foregrip
 * Attaches a high-velocity impact grenade launcher to a semi-automatic or bolt-action rifle (it is always armed unlike in Call of Duty 4: Modern Warfare and Modern Warfare 2.
 * Decreases the recoil of shotguns.

Vehicle Perks

 * Vehicle perks are not available on the Wii version of Call of Duty: World at War. 

Call of Duty: Modern Warfare 2 Perks
In Call of Duty: Modern Warfare 2 the perks are divided into tiers in different way than in Call of Duty 4 and World at War Along with standard perks, players will now be able to unlock improved "Pro" versions of certain perks by fulfilling certain requirements while using them. These "Pro" versions include the original abilities of the perk, but also add new benefits. Many of the perks that were in Call of Duty 4 or 5 that are missing are now either part of a Pro perk, available as weapon attachments, or are now deathstreaks, such as Deep Impact (FMJ), Martyrdom and Double Tap (Rapid Fire). The Overkill perk has been replaced partially by the inclusion of Machine Pistols, Shotguns, and Launchers as secondary weapons and also by the One Man Army and Bling perks. Interestingly, if a Perk Challenge IV is achieved mid-game the "Pro" version will take effect immediately for all classes equipped with that perk. However, the effect of the Pro version is granted after respawning.



First Tier
This tier affects players in terms of Movement Speed (Sleight of Hand/Marathon), Ammunition (Scavenger, One Man Army), and Weapon Attachments (Bling).

Marathon (Unlocked at level 1)
 * Unlimited sprint. (Replaces Extreme Conditioning)
 * Marathon Pro: Climb ladders and objects faster.
 * Unlock Pro: Run 26 miles using marathon.
 * Unlock Emblem: Run 104 miles using marathon

Sleight of Hand (Unlocked at level 1)
 * Faster reloading.
 * Sleight of Hand Pro: Faster aiming down the sight or scope.
 * Unlock Pro: Get 120 kills using Sleight of Hand.
 * Unlock Emblem: Get 750 kills using Sleight of Hand.

Scavenger (Unlocked at level 13)
 * Resupply from dead bodies.
 * Scavenger Pro: Extra mags upon spawning. (Replaces Bandolier)
 * Unlock Pro: Resupply from 100 dead bodies.
 * Unlock Emblem: Resupply from 500 dead bodies.

Bling (Unlocked at level 21)
 * Allows two attachments to primary weapon.
 * Bling Pro: Allows two attachments to secondary weapon.
 * Unlock Pro: Get 200 kills using two weapon attachments
 * Unlock Emblem: Get 900 kills using two weapon attachments.

One Man Army (Unlocked at level 45)
 * Allows class switching ingame. (Replaces secondary weapon)
 * One Man Army Pro: Faster class switching (from 5 seconds to 3).
 * Unlock Pro: Get 120 kills using One Man Army
 * Unlock Emblem: Get 750 kills using One Man Army

Second Tier
This Tier deals with the effects on Damage (Stopping Power/Danger Close), Movement (Lightweight) and bonus effects on Killstreaks (Hardline/Cold-Blooded).

Stopping Power (Unlocked at level 1)
 * Increased bullet damage by 40%.
 * Stopping Power Pro: Increased damage versus vehicles.
 * Unlock Pro: Get 250 kills using Stopping Power
 * Unlock Emblem: Get 1000 kills using Stopping Power

Lightweight (Unlocked at level 1)
 * Move 7% faster.
 * Lightweight Pro: Quicker aiming after sprinting.
 * Unlock Pro: Sprint 30 miles using Lightweight.
 * Unlock Emblem: Sprint 250 miles using Lightweight.

Hardline (Unlocked at level 9)
 * Killstreaks require 1 less kill.
 * Hardline Pro: Deathstreaks require 1 less death.
 * Unlock Pro: Get 40 killstreaks while using Hardline.
 * Unlock Emblem: Get 250 killstreaks while using Hardline.

Cold-Blooded (Unlocked at level 25)
 * Makes the user invisible to enemy thermal optics, and enemy killstreaks. (replaces UAV Jammer and Camouflage)
 * Cold-Blooded Pro: No red crosshair or name when targeted.
 * Unlock Pro: Destroy 40 killstreak rewards using Cold Blooded.
 * Unlock Emblem: Destroy 250 killstreak rewards using Cold-Blooded.

Danger Close (Unlocked at level 33)
 * Increased explosive damage. (replaces Sonic Boom and Fireworks)
 * Danger Close Pro: Increased damage from killstreak rewards.
 * Unlock Pro: 100 kills using explosives while using Danger Close.
 * Unlock Emblem: 500 kills using explosives while using Danger Close.

Third Tier
This tier affects the enemy players equipment (SitRep); and gives players a tactical advantage in combat (Scrambler/Ninja/Steady Aim/Commando/Last Stand).

Commando (Unlocked at level 1)
 * Increased melee lunge distance.
 * Commando Pro: No falling damage.
 * Unlock Pro: 20 melee kills using Commando
 * Unlock Emblem: 100 melee kills using Commando

Steady Aim (Unlocked at level 1)
 * Increased hip-fire accuracy.
 * Steady Aim Pro: Breath can be held longer while aiming down a sniper or thermal scope. (Replaces Iron Lungs)
 * Unlock Pro: 80 hip fire kills using Steady Aim.
 * Unlock Emblem: 500 hip-fire kills using Steady Aim.

Scrambler (Unlocked at level 17)
 * Jams the enemy radar when they are close enough.
 * Scrambler Pro: Delays enemy claymore explosions.
 * Unlock Pro: 50 close range kills using Scrambler.
 * Unlock Emblem: 250 close range kills using Scrambler.

Ninja (Unlocked at level 29)
 * Makes Player invisible to the Heartbeat Sensor.
 * Ninja Pro: 4x quieter footsteps. (Replaces Dead Silence)
 * Unlock Pro: 50 close range kills using Ninja.
 * Unlock Emblem: 250 close range kills using Ninja.

SitRep (Unlocked at level 37)
 * Detect enemy explosives (Claymores, C4, and Grenades) and Tactical Insertions. (Replaces Bomb Squad)
 * SitRep Pro: 4x louder enemy footsteps (Ninja Pro users footsteps remain at 1/4x regular volume).
 * Unlock Pro: Destroy 120 enemy pieces of equipment using SitRep.
 * Unlock Emblem: Destroy 750 enemy pieces of equipment using SitRep.

Last Stand (Unlocked at level 41)


 * Use a pistol while bleeding out prior to death.
 * Last Stand Pro: Use your equipment in Last Stand.
 * Unlock Pro: 20 kills while in Last Stand
 * Unlock Emblem: 100 kills while in Last Stand

Deathstreaks
Copycat (Unlocked at level 4, although is available in default classes) Painkiller (Unlocked at level 6, although available in some default classes) Martyrdom (Unlocked at level 27) Final Stand (Unlocked at level 39, although available in some default classes)
 * Copy the loadout of the person who last killed you during the killcam/second after death
 * Unlock Emblem: Kill the person who last killed you with their loadout
 * 4 deaths without kill
 * Divide all incoming damage by 3
 * Unlock Emblem: Survive over 100 damage
 * 3 deaths without kill
 * Drop a live frag grenade upon death
 * Unlock Emblem: Get one kill with Martyrdom grenade
 * Note: Grenade acts like a normal frag and will roll/be thrown
 * 4 deaths without kill
 * Survive after lethal amount of damage (not headshots, knives or explosions) and use primary/secondary weapons and equipment
 * Unlock Emblem: Survive long enough to stand back up
 * 4 deaths without a kill

Call of Duty: Black Ops
It has been confirmed that all Perks will be unlocked at level 4 and then be available for purchase. It is also confirmed that perks will require more than one set of challenges completed to receive the pro version. The first tier perks define the player's appearance. The perks Call of Duty: Black Ops is confirmed to have are as follows:

Call of Duty: Black Ops (Nintendo DS)
Perks are going to be featured in Call of Duty: Black Ops (Nintendo DS).

Trivia

 * In Call of Duty 4, the Tier 1 perks originally appeared after Tier 3 perks.
 * Tier 1 perks in Call of Duty: Black Ops affect the user's third person player appearance.
 * There were rumors on the internet of a "monkey man" perk on Call of Duty: World at War, allowing the ability to climb trees, although there has never been any evidence of the perk. This rumor likely started because someone climbing a tree in this video can be seen:
 * The monkey man perk was even more convincing after a glitch on Makin was found, allowing the player to jump on top of a tree.
 * If a player unlocks the Pro version of a Perk in Call of Duty: Modern Warfare 2, the Perk will change automatically during the game and so the player won't have to change the perk in the Create-A-Class menu.
 * Pro Perks of Call of Duty: Modern Warfare 2 all have stars as the background of the perk's icon, superimposing the graphic on top of it.
 * If watching someone in the Killcam in Call of Duty: Modern Warfare 2, the text by each perk will not say that the perk is pro unless one has the pro version of each and any respective perk. However the pictures have the star background showing that the perk is pro.
 * Call of Duty: Black Ops for the Nintendo DS will be the first Nintendo DS game to feature perks.
 * A glitch was found for getting Cold-Blooded Pro, in which if a helicopter is shot down by bullets causing it to spin down, every bullet that is shot into the falling helicopter will count as a killstreak destroyed. In order for this to work, the player must have Cold-Blooded while shooting the falling helicopter. This is demonstrated in this video:
 * Modern Warfare 2 and Call of Duty: Black Ops are the only Call of Duty games to feature pro versions of perks.
 * In World at War Tier 1 perks were originally green-colored. The green color for early World at War perks is brighter than color of Call of Duty 4 Tier 3 perks.