Experience Points

Experience Points (or XP for short) are a measurement for how your character is progressing. The player character can earn experience points for killing creatures and solving quests. They are used to determine when a character should gain a new level. Different amounts of experience points are awarded for killing more dangerous creatures. For example, in Fallout 3, killing a deathclaw will get you 50 experience points while killing a radroach will give you 1 experience point. Experience points can also be gained for disarming mines and traps, picking locks, hacking computers, passing speech challenges, doing sections of a quest and in Fallout: New Vegas completing passive challenges like killing a set amount of an enemy. Note that experience points earned are not dependent on your character level, meaning you will get a fixed amount of XP from a certain source regardless of your current level.

Fallout
XP in Fallout is a conventional type similar to Dungeons and Dragons where levels are separated by large amount of experience. For example, you need 1,000 XP to reach level 2, 3,000 for level 3, 6,000 for level 4, and so on.

Fallout 2
Fallout 2's XP system is similar to Fallout's. The only major difference is that leveling requires more XP than in Fallout.

Fallout 3 difficulty modifier
Changing the difficulty level will also change experience gained from killing enemies. On Very Hard you will receive 150% of the regular experience for a kill, while a kill on Very Easy will only yield 50%. For example, a deathclaw is worth 25 XP on Very Easy, 50 XP on Normal and 75 XP on Very Hard.

While this means that you can gain up to three times as much experience from fights on Very Hard compared to Very Easy, this is somewhat compensated by the enemies being much tougher. In the end, it depends on the player's skills and play style which difficulty setting allows for faster leveling. Some people play on Very Easy until their enemy is about to die and then change the difficulty to Very Hard and finish them off to get the XP amount from being on the Very Hard difficulty. Some great places to use this method of gaining XP are Old Olney, Bethesda Ruins, and Mason Dixon Salvage as the enemies there re-spawn every 72 hours (3 in-game days).

XP levels caps at level 20 on all difficulties without the Broken Steel add-on, which increases the cap to 30.

Fallout: New Vegas
Fallout: New Vegas, however, does not reward XP increases based on the level of difficulty selected, despite the manual (PC and X360 version) stating otherwise. This somewhat alleviates the stress encountered while playing the game simultaneously on Hardcore, as it is not necessary to play it on Very Hard. No additional trophy/achievement is awarded to those who play the game on Very Hard nor do you gain bonus XP.

Unlike in Fallout 3, Fallout: New Vegas does award XP for kills made by followers. This increases the usefulness of followers and significantly reduces the hassle of having to compete with them for XP.

Fallout Tactics
XP in Fallout Tactics are given to all party members, regardless of the number of members. The game does not feature difficulty modifiers, however the Tough Guy game mode can provide 100% XP bonus (30% in versions prior to 1.27).

Bugs

 * If you earn experience points, then load an older save right before they register, you will get the points in the older save. An easy way to do this is to kill multiple enemies that deal XP independently and load in between the separate XP gains.
 * It is possible that you can earn XP past where you should have leveled up but the game does not do so. You will hence be stuck at level one. Your XP counter may end up reading something like 600/200, indicating that you should have leveled up 400 XP "ago". This may happen at any level or only from the start of the game. If it happens at the start of the game, beginning a new game fixes this bug, so watch your XP meter closely.
 * If starting a second character, the game will recognize the level 1 character as a continuation of the most recent save (typically a level 30 character). This will make the player unable to level up at some point. This can be fixed by deleting all the save files and starting a new game.
 * This can also be solved if you follow these steps:
 * Start a new game on a different profile that has no save file from the game.
 * Go to memory under system settings and delete the most recent auto save from your bugged profile.
 * Start a new game and the bug should not occur. In the case it does, try the same process but exit the town before 200 XP is reached.
 * If the player earns enough experience to level up twice with the experience earned, then the game will register you as leveling twice, but will not allow you to use additional skill points. Ex. If you completed the quest "Ring-A-Ding-Ding" and earned 1000 XP, and leveled up past level 6 and into 8 you would lose the amount of skillpoints you would have gotten for leveling up to level 7.
 * Fixed in a recent patch.

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