Prior to Call of Duty 4: Modern Warfare, the knife made only incidental appearances, and could never actually be used. Melee attacks in the games prior had used pistol whips and bashes with the stock of the gun. Since Call of Duty 4: Modern Warfare, the knife has been the main melee weapon, triggered by pressing the melee button (default E or V on the PC, clicking the right thumbstick on the Xbox 360, clicking the R3 Button on the PS3, or pressing down on the D-Pad on Wii), as taught in the Missions "F.N.G." and "S.S.D.D."
The knife in Call of Duty 4: Modern Warfare is the only knife featured in the game. However, in Call of Duty: Modern Warfare Remastered, the knife can be replaced by other melee weapons such as the Gravedigger. It possesses a range of 128 inches and has a slight delay between the stab and the actual hit. This is easily observed in multiplayer, where connection issues amplify this. However, it always kills an enemy in one slash or stab, except in Old School multiplayer matches, where two knife attacks are required to kill an enemy player at full health. This two-hit kill knifing also applies when double health is applied on Private Matches. In Campaign, the only NPC appearance of the knife is when Captain Price uses one to execute an Ultranationalist in "Ultimatum."
The knife in Call of Duty: World at War shares the same statistics as its counterpart from Call of Duty 4: Modern Warfare. The "stab" animation is slightly faster and the hit "connects" when the player retracts the knife, instead of slightly after the player punctures the enemy. However, it can be rather inconsistent in its ability to connect and kill enemies. It also makes a very loud and distinguishable swinging noise, especially when performing the "stab" animation (observed from the knifer's point of view), which can easily reveal the knifer's presence.
In Nazi Zombies, the knife is one of the starting weapons received in every map. Killing a zombie with it yields 130 points. The number of knife hits it takes to kill a zombie is generally equal to the round number (with the exception of level 4, in where it only takes 3 hits to kill). Because of the number of hits required to kill and its short reach, by the mid to high levels it is ineffective.
In the Zombies map Der Riese, players can replace their usual knife with the Bowie Knife (near teleporter C, just as the player goes through the courtyard) for 3000 points. Dealing a total of 1150 damage, it allows for one-hit kills up to round 12. The Bowie Knife upgrade remains with the player after being downed and if no one revives him/her, upon respawn, the player keeps the Bowie Knife, making it worth the points spent.
In multiplayer, the Bayonet is an alternative weapon for the knife, and can be attached on all bolt-action rifles (the Kar98k, Springfield, Mosin-Nagant and Arisaka), the M1897 Trench Gun, the M1 Garand, the M1A1 Carbine, and the Type 99 LMG. The Bayonet increases the lunge distance to 200 inches, and can be used to lunge over obstacles, but slows the speed of meleeing. Of the bayonets, all of them are knife or sword bayonets, except for the Mosin Nagant's, which is a spike one instead.
In Call of Duty: Modern Warfare 2, the knife's appearance and range are changed since appearing in Call of Duty 4: Modern Warfare.
The game introduced two new knives apart from the default knife: the Tactical Knife (an attachment for handguns that allows faster knife stab and faster recovery) and the Throwing Knife (a ranged alternative available as Equipment). Besides their multiplayer appearances, they also make appearances in the campaign: the Tactical Knife is always used with the USP.45 (but it is not credited) and the Throwing Knife is used at the end of the game to kill Shepherd.
Also featured in Call of Duty: Modern Warfare 2's multiplayer is the tier 3 perk Commando, which greatly increases the knife's lunge range to 176 inches. This perk is often used with Marathon, Lightweight and the Tactical Knife to create a class capable of using only the knife effectively.
In single player, NPCs are able to knife, although they rarely do it. There are four scripted events where an enemy attacks the player with a knife. The first three occur in "The Only Easy Day... Was Yesterday", "Just Like Old Times" and "Loose Ends," where the player breaches a door and an enemy charges at the player with a knife. The other occurrence is when Shepherd stabs Soap in the chest after knocking him down during "Endgame."
The knife returns in Call of Duty: Black Ops with the same damage as its predecessors. In the Campaign, the knife sees a lot more use in scripted events and has a wider range for different uses, usually when stealth is required. Knifing has an important special effect in Gun Game, demoting players to the previous gun tier if they are knifed. Besides the default knife, there are seven other special variants featured in the game: four in the Campaign, three in Zombies, and one in Multiplayer.
The Ballistic Knife: a secondary weapon that fires the blade out of the grip for a silent, ranged attack (though the attacker still appears on the radar). A regular melee attack with the Ballistic Knife is faster, similar to the Tactical Knife from Call of Duty: Modern Warfare 2.
The Prison Knife: a shiv-like primary weapon briefly wielded during the mission "Vorkuta." It appears to be a piece of metal with black tape around one end.
The knife is used in all levels, and is a one hit kill in campaign and multiplayer. While it can be used in zombies, it is not suggested as it takes one hit by a zombie to down the player. The knife only takes one hit to kill a zombie, no matter what round, but if a zombie is running it takes two hits to kill. If the player uses the ADS function while wielding it, they are able to use it as a throwing knife.
The knife appears again in Call of Duty: Modern Warfare 3 as the standard melee weapon. It seems to have retained many visual aspects when compared to the knife in Call of Duty: Modern Warfare 2. If the enemy player is wearing Juggernaut armor, the knife is not a one hit kill. When using the Melee proficiency on a weapon, the player will knife faster while wielding that weapon. Also, the tactical knife on pistols will make the knife slash out earlier. In campaign, however, Sandman can be seen with a unique folding knife.
The knife now features a "return sweep": if the player swipes the knife and retracts it, the blade can still slash enemies and kill them while being put away. Like its predecessor it can be used as the throwing knife by pressing the ADS button.
The standard knife now has little to no lunge and unlike other installments, it receives no aim assist when stabbing, making it more difficult to use than previous installments. However, the knife can now be used as an individual weapon, the Combat Knife, which has a lunge similar to the knife in previous games. The Combat Knife is used when one slot in Create-A-Class in either Primary or Secondary weapons is empty.
The knife appears in Call of Duty: Ghosts. Unlike previous games, which featured a slash attack, an animation is played out where the player grabs their enemy and stabs their target, slowing the player down and otherwise stopping them in their tracks. It is possible for the attacker to be killed mid-stab. The Combat Knife returns as an individual weapon as well. A gold variant was later added in a patch, in which it is executed with a full slash attack animation instead of the stop-and-stab animtion, allowing players to melee faster when equipped with it.
The knife is the default and only melee weapon in Extinction. It's available for all classes, although the Tank is the only class that can upgrade it's damage up to 200%. It's base damage is high enough to kill a Scout (Extinction) with a single critical hit or with 2 body hits. Unlike in the campaign and multiplayer, it does not have a stabing animation, so the only way of attacking is through normal slashes.
An upgrade called "Electric Knife" can be bought from the armory for 30 Teeth. When a Cryptid is hit, an arc of electricity will jump to nearby enemies, stunning them and dealing minor damage, much like the effects of Stun Ammo. However, the Cryptid that was hit does not get stunned by the electricity. Once used, it has a cooldown of 4 seconds. Because of all of this, it's not a popular upgrade for purchase, as it's only useful on very specific situations, and you can do much better with 30 Teeth.
The knife is only used at certain points of the campaign, whereas in normal circumstances the Exo Melee is used instead:
In "Aftermath", melee attacks are performed with a knife during the stealth part.
At the end of "Manhunt", a knife is used during the confrontation with Hades.
At the start of "Utopia", after the fire sprinkler system is triggered, melee can be briefly done with a knife before saving Ilona, though it is useless, and the waste of time in using it will cause a mission failure.
In "Bio Lab", melee is done with a knife during the cloak part.
In "Captured", if Mitchell's currently equipped weapon is empty, he will drop it and draw a knife (in a similar fashion to the Combat Knife from multiplayer) until he picks up another weapon, due to his disabled prosthetic left arm.
At the end of "Terminus", a knife is used during the confrontation with Irons.
The standard knife is only available in Zombies mode as the standard melee weapon (although it makes some brief scripted appearances in the campaign). In other game modes, the player will melee with the butt of their weapon. In multiplayer, five knife-based weapons are available as standalone melee weapons: they are the Combat Knife, the Butterfly Knife, the Fury's Song, the Carver and the Malice.
The standard knife is only available for use in campaign, Gun Game, Infected and All or Nothing (thus allowing one-hit melee kills); in other circumstances, the weapon butt is used for melee attacks like in Black Ops III. In campaign, the knife is available for use via a suit upgrade, replacing the butt attack. Once again, the Combat Knife is available for use as a stand-alone weapon in usual gameplay.
The knife is used during some scripted parts in the campaign. In the mission Operation Black Flag, the SDF sleeper agent Akeel Min Riah uses a knife to stab himself in order to remove a transponder within him before shortly using the Knife to destroy the transponder. The knife that he drops after dying cannot be obtained by the player. Another instance where the knife is used is when Nick Reyes points a knife at the dying Salen Kotch during the same mission, at which point he can use it to stab him or let him succumb to his wounds.
The ending knife in Call of Duty: Modern Warfare 2 has less damage than the original knife, and can hit through riot shields. This is noticeable in the level Museum, by using the console command give ending_knife.
On Modern Warfare 3, if the final KillCam is with a knife, a blood splatter will appear out of nowhere once the knife leaves the enemy's body.
On Modern Warfare 3, the Infinity Ward logo and name is writen on the knife.