Each appears in the form of an item that, when picked up, provides the player with an additional weapon with which to attack opponents, as well as some ammo for the corresponding weapon. Normally, weapons are either placed in levels by the designers or dropped by dying opponents (such as zombies in the Doom games). Once a weapon is picked up during a game, any additional weapons of the same type will count only as ammunition.
Each weapon has advantages and disadvantages when used against a given monster. Broadly speaking, however, the larger weapons do more damage but are harder to find and harder to keep stocked with ammunition. In each game, one or two weak weapons require no ammo and are therefore always available.
No matter which weapon a player is wielding, his sprite (in multiplayer games, or source ports with a "chasecam" mode) is perpetually shown to be armed with the same generic assault rifle as the former humans; a weapon similar to this, although presumably intended to be the chaingun, appears on the game's title screen.
Early alpha releases of Doom armed the player with a semi-automatic rifle, optionally equipped with a double-bladed bayonet as a melee weapon. This weapon was not included in the finished game. The rifle sprites revealed the player character to be wearing elbow pads, which can still be seen on the player character sprite. A few other weapons were also included in the alpha versions.
- Fists: Extremely basic close-range weapon. Never runs out of ammo, but only about as powerful as a pistol shot; normally used only as a last resort or with a berserk powerup.
- Chainsaw: Does damage like the normal fist, but four times faster.
- Pistol: The default long-range weapon. Almost entirely useless against anything stronger than a zombieman or shotgun guy.
- Shotgun: A good general-purpose weapon capable of dealing medium-high damage, especially at close range. Reload time is slightly longer than normal.
- Chaingun: Very good against large crowds of small monsters or single large monsters, but its rapid rate of fire can quickly deplete its ammo supply.
- Rocket launcher: Fires explosive rockets. Does a lot of damage, but can also seriously hurt the player if used indiscriminately at close range.
- Plasma gun: Shoots pulses of blue-hot plasma at high speed, which can take down groups of incoming enemies easily — if aimed properly.
- BFG9000: The "Big Fucking Gun." Somewhat counter-intuitive to operate at first, but kills almost any monster in one shot.
The chaingun and pistol both use the same ammo type (bullets), so using either will also affect the other in ammo. The shotgun and super shotgun (see below) also share their ammo pool (shells) as do the plasma gun and BFG9000, which both use cells.
All of the above weapons, with the addition of:
- Super shotgun: A double-barrelled, sawed-off shotgun which takes even longer to reload, but at close range is even more deadly than the regular shotgun.
All of the above weapons, with the addition of:
- Unmaker: A demonic laser gun which rapidly fires paralyzing red slugs but can be upgraded in firepower by the collection of Demon Keys. The red slugs stun monsters, temporarily rendering them unable to attack for about 2-3 seconds. Monsters are still able to move, however.
All of Doom's weapons are present in some form in Doom 3, with the addition of:
- Flashlight: Not a weapon per se, but it doubles as a crude club.
- Machine gun: Similar to the chaingun, but not as powerful (more accurate though).
- Grenades: Standard thrown fragmentation grenade on a timer.
- Soul cube: Charged by killing monsters, and then released as a powerful seeking weapon that transfers health from the monster it hits to the player.
The Doom 3: Resurrection of Evil expansion pack contains all of Doom 3's weapons except the Soul Cube and the Chainsaw, but introduces a version of the gravity gun called the Grabber, and includes the super shotgun (as in Doom II). It also introduces an artifact somewhat like the Soul Cube.
All of Doom 2's weapons except for the pistol, with the addition of:
- Heavy Assault Rifle: A machine gun like rifle that can have a missile pod attached to it.
Heretic's weapons are more or less equivalents of Doom's weapons, except for the Firemace. Also, each weapon that uses ammunition has its own kind rather than sharing ammo like some Doom weapons. The Doom equivalent of each Heretic weapon is listed in parentheses below:
- Staff (Fists)
- Gauntlets of the Necromancer (Chainsaw)
- Elven Wand (Pistol)
- Ethereal Crossbow (Shotgun)
- Dragon Claw (Chaingun)
- Hellstaff (Plasma gun)
- Phoenix Rod (Rocket launcher)
- Punch Dagger
- Assault Gun
- Mini-Missile Launcher
- Grenade Launcher
- The Sigil of the One God