Prior to Call of Duty 4: Modern Warfare, there was no standard melee weapon. In Call of Duty and Call of Duty: United Offensive, the player was able to melee enemies with the butt of their primary weapon or pistol whip with their handgun. In addition, players could also melee with their grenades, satchel charges, and binoculars. However, enemies were difficult to kill because of the short range, inaccuracy, and low damage of many weapons.
Call of Duty 2 limited the players' melee attack to only their primary weapons and pistols. Call of Duty 3 brought back the ability for the player to melee with whatever weapon or equipment they had. The player still couldn't melee with a knife though, and were only able to use it once in the mission "The Crossroads" during a scripted CQC event.
Call of Duty 4: Modern Warfare was the first game where the player was able to melee with a knife, as well as the first to completely remove the ability to melee with whatever the player was holding. The knife has served as the default melee weapon for all games succeeding Modern Warfare, however, Call of Duty: World at War and later games also debuted alternate melee weapons the player could attack with. Many of these are unable to be used in multiplayer, rather they are only usable in Campaign and Zombies mode.
Additionally, in the campaign mission "Captured", the player can melee by pistol whipping with the Atlas 45, the SN6 and the AMR9. However, these melee attacks aren't one-hit kill, unlike other melee weapons.
Unlike previous games, players will not melee with a knife if they have their other weapon out, instead they will melee with the butt of their weapon or pistol whip. In order to melee with another weapon, they will have to select the combat knife, or have no weapons whatsoever, which will give the player Fists. Melee with one's gun can kill in one to the sides or rear, but requires two hits to the front. The fists take two hits to kill regardless. Furthermore, as of an update, there is now an actual weapon class for eighteen standalone melee weapons in multiplayer.