In Quake, there are 4 difficulty levels. These are Easy (Meant for little kids and grandmas), Normal (Most people should start Quake at Medium skill), Hard (Here at id, we play hard skill, and we think you should too, when you're ready) and Nightmare (This is so bad that the entry is hidden, so people don't wander in by accident. If you find it, don't say we didn't warn you). The first three difficulty levels can be chosen from the very first map when starting a new game. Hard is, of course, the hardest difficulty to choose as the player must jump over a pit of Lava. Lava balls shoot upwards and if the player gets hit by one, he/she will stall in the air and fall in the lava, resulting in death.
Unlike the other difficulty levels, Nightmare is hidden and can only be obtained if the player knows where to look. To find Nightmare, go through any portal at the start and then walk up the steps leading to Episode 4. Jump into the Water and then go backwards from where the player entered before falling out the bottom. If done right, the player will land on a wooden beam. If he/she misses, then look up and he/she will see the beam and know where to fall on the next try. Follow the beam to the opening in the wall and the player will find a lava portal. Go through this portal to play the Nightmare difficulty. Nightmare difficulty makes the monsters' attacks a lot faster. This is mostly seen in the Ogres, who now fire their grenades a lot faster and the Shamblers, whose charge up time to launch ranged attacks is much quicker.
In Quake II, there are again 4 difficulty levels which are pretty much the same as Quake's but with slightly different names. They are Easy, Medium, Hard and Hard+. These are chosen from the Singleplayer menu, and choosing one will begin a new game in that difficulty.
Hard+ (which is the same as Nightmare in Quake) is again hidden and can only be selected by typing "Skill 3" and then "Map Base1" in the console. This will put the player in the very first level with the difficulty of Hard+. Hard+ makes the enemies' attacks faster and there is a much lower chance of any of the enemies flinching. This makes Gunners and Icarus much more difficult.
In Quake 3, there are five difficulty levels instead of the normal four. All of them can be selected when starting any Singleplayer map. The difficulty levels are "I Can Win", "Bring It On", "Hurt Me Plenty", "Hardcore" and "Nightmare!". Depending on each difficulty will depend on the skill of the bots in each map.
The easier difficulties will make the bots inaccurate, handicapped, ignoring the power-ups, very slow to react and turn; whilst the harder difficulties will make the bots fast as a pro-player, no handicap, gathering power-ups whenever available and more accurate than humanly possible, presenting a true challenge.
On "I Can Win", "Bring It On" and "Hurt Me Plenty" difficulties bots are handicapped; respectively 50%, 70% and 90%. They can only deal a certain percentage of damage and carry limited amount of health and armor, anything above that limit will decay slowly. At "Hardcore" and "Nightmare!" difficulty levels, bots have no handicap and significantly faster react time and turning speed. Forces player to react faster, aim accurately and time item spawns to stay alive.
In Quake 4, there are four difficulties, each one represented by a military rank, the higher the rank is, the more difficult it will be. The difficulties are "Private", "Corporal", "Lieutenant" and "General". The higher a difficulty is, the harder the enemies will be to kill, you will take higher damage from attacks, and enemies' health will be higher. A difference from most other Quake games, it's not necesary to unlock or discover the hardest difficulty, you may select from the starting menu.