Perks

Perks are Call of Duty 4: Modern Warfare 's, Call of Duty: World at War 's, and Call of Duty: Modern Warfare 2 's multiplayer modifiers 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.

List of perks in 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 one time. The tiers are colour coded blue, red, and green and therefore only one of each can be equipped.

Blue Tier
These perks usually deal with a player's equipment.

Bomb Squad
 * Warns players of enemy explosives. (Unlocked at level 14)

C4 x2
 * Two blocks of Composition-4. (Available at level 1)

Claymore x2
 * Two Claymore mines. (Unlocked at level 23)

RPG-7 x2
 * Two RPG-7s. (Available at level 1)

Special Grenades x3
 * Three Flash Grenades or three Stun Grenades (Available at level 1)

Bandolier
 * Extra ammunition. (Unlocked at level 32)

Frag x3
 * Three Fragmentation Grenades. (Unlocked at level 41)

The following perks cannot be directly selected. Adding the corresponding attachment to your Primary Weapon will disallow the selection of any other Perk 1, and so has the effect of being a Perk 1.

M203 Grenade Launcher/ GP-25 Grenade Launcher
 * Attaches a high-velocity impact grenade launcher to an assault rifle.

Foregrip
 * Decreases the recoil of light machine guns and shotguns.

Red Tier
These second tier perks generally affect the way players or their weapons operate.

Juggernaut
 * Increases health.

Sleight of Hand
 * Faster reload. (Unlocked at level 20)

Sonic Boom
 * Increased explosive damage.

Stopping Power
 * Increased bullet damage.

Double Tap
 * Increases rate of fire. (Unlocked at level 29)

UAV Jammer
 * Makes the player unseen to enemy UAVs. (Unlocked at level 11)

Overkill
 * Carry two primary weapons. (Unlocked at level 38)

Green Tier
The last tier; these act as modifiers that change the physiques of players or what they do before and after dying (with the exception of Deep Impact).

Deep Impact
 * Increased bullet penetration.

Extreme Conditioning
 * Sprint for longer distances.

Last Stand
 * Pull out your pistol before dying. (Unlocked at level 8)

Martyrdom
 * Drop a live grenade when killed. (Unlocked at level 17)

Steady Aim
 * Increases hip-fire accuracy.

Dead Silence
 * Reduces the footstep noise. (Unlocked at level 44)

Iron Lungs
 * More steadying time for sniping. (Unlocked at level 26)

Eavesdrop
 * Hear enemy voice chat. (Unlocked at level 35)

First Tier
Special Grenades x3
 * Gives the player 3 special grenades excluding smoke grenades.

Satchel Charge x2
 * Similar to the C4 in Call of Duty 4.

M9A1 Bazooka x2
 * Similar to the RPG in Call of Duty 4, perhaps more accurate.

Bomb Squad
 * Same as it was in Call of Duty 4, you can see where enemy Satchels and Bouncing Betties are located.

Bouncing Betty x2
 * Similar to the Claymore in Call of Duty 4, mines that are planted in the ground.

Bandolier
 * Increases initial ammo for your selected weapons.

Primary Grenades x2
 * 2 frag grenades, sticky grenades, or Molotov Cocktails.

M2 Flamethrower
 * The player is equipped with a flamethrower.

Second Tier
Stopping Power
 * Weapon damage is increased by 40%.

Fireworks
 * Explosive damage is increased.

Flak Jacket
 * Explosive damage dealt to the player is decreased.

Gas Mask
 * The effects of Tabun gas are decreased.

Juggernaut
 * Damage from bullets is decreased.

Camouflage
 * The player doesn't get exposed by recon planes. Identical to UAV Jammer.

Sleight of Hand
 * Reloading is quicker.

Shades
 * Reduces the effect of signal flares on the player's vision. Not available on the Wii version.

Double Tap
 * Increases the player's weapons' rate of fire.

Overkill
 * Allows the player to wield 2 primary weapons (meaning no pistols). Same as Call of Duty 4.

Third Tier
Deep Impact
 * Gives the user's weapon more penetration power.

Extreme Conditioning
 * Gives the player the ability to sprint longer.

Steady Aim
 * More accuracy when firing from the hip. Note: this does not help with aiming down the sights.

Toss Back
 * When tossing back a grenade, it resets the grenade fuse.

Second Chance
 * Almost the same thing as Last Stand in Call of Duty 4 except you survive much longer and another player who has Second Chance will be able to revive them.

Martyrdom
 * When the player is killed, they drop a live grenade.

Fireproof
 * Damage from fire done to the player is reduced.

Dead Silence
 * The player barely makes, if any, noise when walking or sprinting.

Iron Lungs Reconnaissance
 * Gives the user the ability to hold their breath with a sniper rifle for 10 seconds instead of 4.
 * Gives the player the ability to see all artillery, dogs, and tanks on the map.

Vehicle Perks
Water Cooler
 * Decreased turret overheating.

Greased Bearings
 * Increased turret rotation speed.

Leadfoot
 * Increases the speed of the tank.

Ordnance Training
 * Boosts the firing rate of the tank's main firing cannon.

Coaxial Machine Gun
 * Allows the driver to use a coaxial machine gun only in the front of the tank.

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

List of Perks in Modern Warfare 2
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, or available as weapon attachments, such as Deep Impact (FMJ), and Double Tap (Rapid Fire). Interestingly, if a Perk Challenge IV is achieved mid-game the "Pro" version will take effect immediately for all classes equipped with that perk.

First Tier
Marathon
 * Unlimited sprint
 * Marathon Pro: Ability to vault over obstacles/cover and climb ladders faster

Sleight of Hand
 * Faster reloading
 * Sleight of Hand Pro: Faster ADS

Scavenger
 * Full resupply from dead enemies
 * Scavenger Pro: Spawn with extra ammo. (Replaces Bandolier.)

Bling
 * 2 primary weapon attachments
 * Bling Pro: 2 secondary weapon attachments

One Man Army
 * Swap classes at any time (takes the place of your secondary weapon)
 * One Man Army Pro: Faster class swap

Second Tier
Stopping Power
 * Increased bullet damage
 * Stopping Power Pro: Increased damage versus vehicles.

Lightweight
 * Move faster
 * Lightweight Pro: Quick aim after sprinting.

Hardline
 * Killstreaks require 1 less kill
 * Hardline Pro: Deathstreaks require 1 less death.

Cold-Blooded
 * Makes the user invisible to enemy Sentry Guns, UAV's, thermal weaponry, enemy helicopters and enemy controlled killstreaks (such as AC-130's). Possesses the powers of UAV Jammer and Camouflage.
 * Cold-blooded Pro: No red crosshair or name when targeted.

Danger Close
 * Increased explosive damage like Sonic Boom and Fireworks.
 * Danger Close Pro: Increased explosive damage from killstreaks.

Third Tier
Commando
 * Increased melee lunge distance
 * Commando Pro: No fall damage taken.

Steady Aim
 * Increased hip-fire accuracy
 * Steady Aim Pro: Breath can be held longer. (Replaces Iron Lungs)

Scrambler
 * Jams the enemy radar when they are close enough
 * Scrambler Pro: Delays enemy claymore explosions.

Ninja
 * Makes Player invisible to the Heartbeat Sensor
 * Ninja Pro: 4x quieter footsteps. (Replaces Dead Silence)

SitRep
 * Detect enemy explosives and Tactical Insertions. (Replaces Bomb Squad)
 * SitRep Pro: 4x louder enemy footsteps, although it doesn't cancel an enemy's Ninja Pro.

Last Stand
 * Use a pistol while bleeding out prior to death.
 * Last Stand Pro: Use your equipment in last stand.