Perks

Perks are modifiers featured in the multiplayer aspects of Call of Duty 4: Modern Warfare, Call of Duty: World at War, Call of Duty: Modern Warfare 2, Call of Duty: Black Ops, Call of Duty: Black Ops (Nintendo DS), Call of Duty: Modern Warfare 3, Call of Duty: Black Ops II and have been confirmed in Call of Duty: Ghosts. They passively affect gameplay, allowing further customization of classes. They affect available equipment, effectiveness of the player's equipment, the speed the player's equipment can be used, the usefulness of the enemy's equipment, and the physical properties of the player's character.

Call of Duty 4: Modern Warfare
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 once.

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

Vehicle perks, Shades, Iron Lungs and Reconnaissance are not available on the Wii version of Call of Duty: World at War.

Call of Duty: Modern Warfare 2
In Call of Duty: Modern Warfare 2, the perks are divided into tiers in different ways 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 and World at War that are missing are now either part of a Pro perk, available as weapon attachments, or are now deathstreaks, such as Deep Impact (FMJ), Bandolier (Scavenger Pro), and Martyrdom. 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. Up to three perks, one of each tier, can be equipped at one time. The tiers are color coded blue, red, and green; only one of each can be equipped.

Call of Duty: Black Ops
All Perks are unlocked for purchase at level 4. Unlocking Pro Perks requires three sets of challenges related to the standard version to be completed, as well as the purchase of the ability for 2000 CoD Points and the 3000 cost for the upgrade to its Pro version. The appearance of the Pro perks are done in a similar fashion to the ones in Modern Warfare 2, where the background becomes a star with the perk graphic superimposed over top of it.

First tier perks designate player appearance and provide constant passive effects. Second tier perks enhance weapon performance. Third tier perks can be either active or passive and are generally more context-sensitive than first tier perks. The perks featured in Black Ops are as follows.

Call of Duty: Black Ops (Nintendo DS) and Modern Warfare 3: Defiance
Perks are featured in Call of Duty: Black Ops (Nintendo DS) and can be selected during respawn and at the menu.

The perks are unlocked when the player reaches various multiplayer levels. The Perks are:

These perks return once again in Call of Duty: Modern Warfare 3: Defiance, but are now sorted into tiers. So unlike Black Ops DS where the player can pick any perk to equip in their perk slots. Players are now restricted into using perks in their allocated tier.

Call of Duty: Ghosts
Perks have been confirmed in Call of Duty: Ghosts. A player starts with 8 'points' to distribute between 35 perks. If they remove their secondary, they can have 11 'points'. To balance this out, some perks are worth more than others. They can be worth 1, 2, 3, 4, or 5 points. There are 7 categories, Speed, Handling, Stealth, Awareness, Resistance, Equipment, and Elite.

Some early info has come out about these perks (these may not be completely correct)

Speed
Dexterity (1): Allows weapons to be ready faster after sprinting

Sleight of Hand (2): Allows player to reload ammunition faster

Agility (2): Allows player to move faster (similar to lightweight)

Marathon (2): Grants players unlimited sprint

Stalker (2): Allows players to move faster while aiming down sight

Handling
Strong-Arm (1): Allows players to throw equipment farther and decrease the cook time of grenades.

On the Go (2):  Allows the player to reload while sprinting

Reflex (2): Allows players to swap weapons and equipment faster

Steady Aim (2): Reduces a weapons hipfire spread

Quickdraw (2): Allows players to aim down sight faster

Stealth
Takedown (1): When a player kills an enemy, enemy players will not see the skull above their dead teammates

Blind Eye (2): When used, players are undetectable by air support, Oracle systems, and sentries

Off the Grid (2): It grants the player immunity to enemy SAT COMs and radar pings

Dead Silence (2): Silences players footsteps (May affect Recon)

Cold-Blooded (2): When equipped, players are undetectable by enemy player-controller killstreaks

Awareness
Recon (1): Explosive damage paints targets on the minimap

Scavenger (2): Replenishes ammo and grenades from fallen enemies (like MW3)

SitRep (2): Players can detect enemy equipment and explosives nearby

Eavesdrop (2): Allows players to hear enemies louder

Wiretap (2): Allows the player to use enemy SAT COMs as if they were on the friendly team

Resistance
Resilience (1): Negates fall damage taken by the player

Painkiller (2): Allows the player to regenerate their health faster after taking damage

Tac Resist (2): Reduces the effects of tactical grenades

Stopper (2): Reduces flinch when shot

Blast Shield (2): Provides increased resistance to explosive damage

Equipment
Extra Tactical (1): Allows the player to carry an extra tactical grenade

Extra Lethal (2): Allows the player to carry an extra lethal grenade

Fully Loaded (2): Allows players to start off with extra magazines, similar to Bandolier from previous games

Extra Attachment (2): Allows the player to equip a third attachment to their primary weapon

Danger Close (3): Increased explosive damage

Elite
Gambler (1): When spawned, the perk will be replaced by a random perk. No stacking like perks

Hardline (2): Pointstreaks require one less point to obtain

Ping (2): When a player kills an enemy, the perk will notify the player if there are any enemies in the vicinity

Overkill (3): Allows player to carry two primary weapons

Deadeye (5): The probability of extra bullet damage granted increases as the player goes on a longer killstreak.

Trivia

 * In Call of Duty 4, the Tier 1 perks originally appeared after Tier 3 perks.
 * In Call of Duty 4 and Call of Duty: World at War, it is possible to empty the player's Tier 1 perk slot by attaching a grenade launcher or a grip to a weapon and then removing it.
 * If a player unlocks the Pro version of a Perk in Call of Duty: Modern Warfare 2 and Call of Duty: Modern Warfare 3, the Perk will change automatically during the game and so the player will not have to change the perk in the Create-A-Class menu. In Black Ops, the player has to buy it.
 * Pro perks all have stars as the background of the perks 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 the killed player has the Pro version of each respective perk. However, the pictures have the star background showing that the perk is Pro.
 * Call of Duty: Black Ops (Nintendo DS) is the first Nintendo DS Call of Duty game to feature 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. Also, the bright green color was used for cut squad perks such as All Seeing Eye and Commanding Squad Leader.
 * World at War has the most perks in all of the Call of Duty games with 33 perks.
 * Dead Man's Hand was originally going to appear in Call of Duty: Modern Warfare 2, but was cut for unknown reasons.
 * Originally, Copycat had a black cat as its picture, as seen in the official strategy guide, but was changed to the mask later in game development.
 * Call of Duty: Black Ops II is the first Call of Duty game since Call of Duty: World at War to feature no Pro Perks.
 * Call of Duty: Black Ops II is the first game to include Wildcards that allow the player to have more than three perks in total and more than one perk in a tier.
 * In Call of Duty 4, Call of Duty: Modern Warfare 2 and Call of Duty: Black Ops II  there is an extra Perk in tier 3.
 * In Call of Duty: Black Ops and Call of Duty: Black Ops II, perks in each tier have a specific shape. Tier 1 is in the shape of a shield, Tier 2 is an up-side-down pentagon with Tier 3 being an arrowhead.
 * Both Call of Duty 4: Modern Warfare and Call of Duty: Modern Warfare 2 have perk icons that are curved in the sides, while in Call of Duty: Modern Warfare 3 the perk icons appear just as squares.