Melee

In the Call of Duty series, the melee attack is an close-quarters attack triggered using a different key or button than the fire button.

Pre-Call of Duty 4: Modern Warfare
Before Call of Duty 4: Modern Warfare, melee attacks consisted of pistol whips and smacks with the gunbutt. While the latter was capable of an instant kill, historically the former has not been able to do so.

In Call of Duty and United Offensive, when all equipment had to be equipped, it was possible to melee with every piece of equipment, including grenades, smoke grenades, satchel charges, and binoculars. Melee damage for these other pieces of equipment had the same melee damage as a pistol (50), and as such were used only for humiliation kills. Melee damage from primary guns, on the other hand, was more substantial, and a melee hit to the head, body, or back was enough to kill. However, in practice, this was rarely achieved. For one, melee attacks had extremely short range (about 3 feet). In addition, melee attacks connected inconsistently, and when they did connect, they often connected on the shoulders or legs, where they would be negatively affected by multipliers. Because of these, melee attacks did not see much use. The AWE (Additional War Effects) mod attempted to make the melee attack more viable by shellshocking a player who had been meleed, as well as causing a bleed effect.

Call of Duty 2 made several changes to melee damage. Because equipment was no longer selectable, it was no longer possible to melee with grenades or the like. Melee damage for all non-pistol weapons was made consistent (150 damage), making it fatal no matter where the melee hit connected. Connection was improved also, although this may have been due to the enlargement of player hitboxes in Call of Duty 2. Pistols still continued to do 50 damage. Because melee hits were now effectively instant kills, they could be utilized more often. However, melee attacks still had very short range and no lunge.

Post-Call of Duty 4: Modern Warfare
Call of Duty 4: Modern Warfare brought a sweeping change to the melee attack in the form of the knife. Each weapon had the same melee attack, a slash or stab with a knife capable of dealing an instant kill. In addition, the range was increased to about 5 feet, as a slight lunge was created. The ability to sprint, use perks, and deal an instant kill melee while carrying around light weaponry (namely pistols) allowed skilled players to play entire games using the only the knife.

Call of Duty: World at War saw a sudden nerf to the knife; although it was still an instant kill, its connection was extremely poor, and the range was decreased. However, World at War did see the proliferation of multiple melee weapons. The bayonet, which could be attached to rifles, allowed for melee attacks at longer ranges (about 6 feet). The Bowie Knife, available only on Der Riese, increased knife damage from 200 to 1000, allowing one-hit-kills against the zombies for 12 rounds.

Call of Duty: Modern Warfare 2 saw the return of Modern Warfare's knife, the same in range and damage. In World at War's spirit, though, it added another melee weapon, the Tactical Knife, which allowed for faster melee speed and recovery. Modern Warfare 2 also featured several perks conducive to knife-only plays, Commando being the most infamous for doubling melee range from 3.5 feet to 10 feet.

Call of Duty: Black Ops is set to see the return of the knife.