Skill level

Each game skill level provides general difficulty settings that allow a greater variety of players to enjoy the game, allowing novices to face fewer opponents even with advantages for the player, and giving those who have mastered the game a challenge against many monsters, possibly with enhanced aggressiveness. Each skill level bears a colorful name suiting the character of the particular game being played and can be selected when the game starts or through additional options, particularly the -skill parameter.

Picking "Nightmare" will trigger the game telling the player that said difficulty "isn't even remotely fair."

Doom, Doom II and Final Doom skill levels

 * 1) This is the name or tag for each skill level according to the source code.
 * 2) Each thing placed in a level (except for player starts) has cumulative bit flags (easy, normal, hard) marking on what skill levels it appears. The difference mainly amounts to larger and tougher behavior of monsters on the harder skill levels, but may also account for changes in the location of certain things. For example, in E3M6: Mt. Erebus the blue key is located in different places depending on the skill level.
 * 3) Double means twice as much ammo is added to the player's current total when a corresponding item is picked up. For example, a rocket will provide 2 rockets, 4 shotgun shells will be counted as 8 shells, an energy cell will provide 40 units, and a box of bullets will provide 100 bullets instead of the usual 50.
 * 4) All cheat codes except idclev and iddt are disabled.
 * 5) Monsters attack more rapidly, demons and spectres move faster and several monster fireball types fly faster.

Heretic skill levels
See the Doom and Doom II table above for specific notes, as there are no differences in general, except that +50% means one and a half times as much ammo is added to the player's current total when a corresponding item is picked up, and that monsters do not respawn on the "Black plague possesses thee" skill level like they do in Doom's "Nightmare!".

Hexen skill levels
See the Doom and Doom II table above for specific notes, as there are no differences in general, except that monsters do not respawn on the hardest skill level (as in Heretic).

Strife skill levels
See the Doom and Doom II table above for specific notes, as there are no differences in general.

Doom (2016) and Doom Eternal skill levels
In DOOM Eternal, the difficulties do not affect standard enemy health, but it does impact boss health.

Doom Eternal also has an "Extra Life Mode", where a game ends if all Extra Lives are used up, like in an old-style video game. Upon selecting Extra Life Mode, the player then selects one of the difficulty levels, so the mode can be in any difficulty level except Ultra-Nightmare.