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.
|Skill||Menu name||Source code1||Thing placement2||Ammo3||Special|
|1||I'm too young to die.||sk_baby||The same as in sk_easy||Double||Players take half normal damage|
|2||Hey, not too rough.||sk_easy||Least numbers or strength||Normal|
|3||Hurt me plenty.||sk_medium||Default numbers or strength||Normal||Used unless otherwise defined by the user|
|4||Ultra-Violence.||sk_hard||Greatest numbers or strength||Normal|
|5||Nightmare!||sk_nightmare||The same as in sk_hard||Double||No cheats4, fast5 and respawning monsters|
- This is the name or tag for each skill level according to the source code.
- 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 comportments 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.
- 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, and a box of bullets will provide 100 bullets instead of the usual 50.
- All cheat codes except idclev and iddt are disabled. Some source ports may change this or have internal workarounds.
- Monsters attack more rapidly, demons and spectres move faster and several monster fireball types fly faster.
|Skill||Menu name||Source code1||Thing placement2||Ammo3||Special|
|1||thou needeth a wet-nurse||sk_baby||The same as in sk_easy||+50%||Players take half normal damage, keys are shown on the automap|
|2||yellowbellies-r-us||sk_easy||Least numbers or strength||Normal|
|3||bringest them oneth||sk_medium||Default numbers or strength||Normal||Used unless otherwise defined by the user|
|4||thou art a smite-meister||sk_hard||Greatest numbers or strength||Normal|
|5||black plague possesses thee||sk_nightmare||The same as in sk_hard||+50%||No cheats4, fast monsters5|
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.
|Source code1||Thing placement2||Special|
|1||squire||altar boy/novice||apprentice||sk_baby||The same as in sk_easy||Players take half normal damage|
|2||knight||acolyte/missionary||enchanter||sk_easy||Least numbers or strength|
|3||warrior||priest/acolyte||sorceror||sk_medium||Default numbers or strength||Used unless otherwise defined by the user|
|4||berserker||cardinal/initiate||warlock||sk_hard||Greatest numbers or strength|
|5||titan||pope/mystic||archimage||sk_nightmare||The same as in sk_hard||No cheats4, fast monsters5|
The alternate righthand titles for the Cleric are used on the N64 port.
See the Doom and Doom II table above for specific notes, as there are no differences in general.
|Skill||Menu name||Thing placement||Ammo||Special|
|1||Training||The same as in skill 2||Double||Players take half normal damage; monsters move at half-speed1|
|2||Rookie||Least numbers or strength||Normal|
|3||Veteran||Default numbers or strength||Normal||Used unless otherwise defined by the user|
|4||Elite||Greatest numbers or strength||Normal|
|5||Bloodbath||The same as in skill 4||Double||No cheats; fast monsters2; Robots, Templars, and turrets respawn|
- Acolytes and stalkers move, attack, and react to pain half as fast. Reavers and ceiling turrets attack half as fast. Crusaders attack and react to pain half as fast. The Bishop's homing missiles move twice normal speed (see Bishop's missiles have reversed skill properties).
- All monsters attack aggressively. Acolytes move, attack, and react to pain twice as fast. The Bishop's homing missiles move at half-speed.
- For other relevant notes, see the Doom and Doom II table above.
|Skill||Menu name||Thing placement|
|1||Be Gentle!||Least numbers or strength of opposition|
|2||Bring it on!||Lesser numbers or strength of opposition|
|3||I own Doom!||Greater numbers or strength of opposition|
|4||Watch me die!||Greatest numbers or strength of opposition|
 Non-Doom engine games
|0||Recruit||The Marine takes 75% of the base damage. Slightly fewer monsters than usual are encountered.|
|1||Marine||The Marine takes 90% of the base damage.|
|2||Veteran||The Marine takes 170% of the base damage.|
|3||Nightmare||The Marine takes 350% of the base damage. All medikits are removed from the game, and the player's health steadily drains until it's 25 or below. To restore health, the player must use the Soul Cube (Artifact in RoE), or health stations. Player starts the game with respective artifact in their possession. Number of monsters is the same as on Veteran skill. This skill level must be unlocked by first completing the game on any other skill level.|
- In addition to the five Doom standard skill levels, in vanilla Doom a skill level of 0 can also be specified, but only using the -skill parameter. Doing this empties maps of all things apart from the player starts. This does not work in many source ports. It also renders many levels that have boss trigger effects, require keys or use teleporters impossible to finish without the aid of cheat codes.
- Early versions of Doom did not support "Nightmare!" skill. The initial mail-order releases were version 1.1 and only included the first four skill levels. The widely circulated version 1.2 added "Nightmare!" skill support. "Nightmare!" was added with the characteristic dark humor of the Doom games in response to any comments saying the game was too easy on "Ultra-Violence." After selecting "Nightmare!" from the main menu, the player is asked "Are you sure? This skill level isn't even remotely fair." The player must then confirm that he wants to play on Nightmare mode, a step which is unique to this skill level.
- The name of Doom's fourth skill level, "Ultra-Violence", comes from a phrase in Anthony Burgess's 1962 novel A Clockwork Orange, via Stanley Kubrick's 1971 film version .
- In the Doom alphas, "I'm too Young to Die" was called "I Just Want to Kill", hence the name M_JKILL for the graphic for skill 1.
- Doom's "I'm Too Young To Die" skill level was re-named "I am a wimp" for Atari Jaguar, 3DO, Sony PlayStation and Sega Saturn versions.
- Some skilled players have created new challenges by defining custom rules, such as pacifist (not hurting monsters) and Tyson (fists, pistol, and chainsaw only).