Experience (PHB)

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After a player's character has bravely set out and survived his first adventure, the player will have experienced the entertainment of role-playing games. But what will the character have gained? If the character never improves, he will never be able to survive, let alone overcome the powerful dangers that fill the AD&D game worlds.
Fortunately, this isn't the case. Every time a character goes on an adventure he learns something. He may learn a little more about his physical limits, encounter a creature he has never seen before, try a spell as yet unused, or discover a new peculiarity of nature. Indeed, not all his learning experience need be positive. After blowing up half his party with a poorly placed fireball, a wizard may (though there is no guarantee) learn to pay more attention to ranges and areas of effect. After charging a basilisk, a fighter may learn that caution is a better tactic for dealing with the beast (provided the other characters can change him from stone back to flesh). Regardless of the method, the character has managed to learn something.
Some of the information and skills learned in the game can be applied directly in play. When a wizard toasts his friends with a badly cast fireball, the player learns to pay more attention to the area of effect of a fireball. Though the player made the mistake and his character only carried out the actions, the player's friends will also learn to keep their characters well away from his.
The reward for this type of learning is direct and immediate. The characters benefit because each of the players has a better understanding of what to do or where to go
However, a character also improves by increasing his power. Although the player can improve his play, he cannot arbitrarily give his character more hit points, more spells, or a better chance to hit with an attack. These gains are made by earning experience points (XP).
An experience point is a concrete measure of a character's improvement. It represents a host of abstract factors: increased confidence, physical exercise, insight, and on-the-job training. When a character earns enough experience points to advance to the next experience level, these abstract factors translate into a measurable improvement in the abilities of the character. Just what areas improve and how quickly improvement occurs all depend on the character's class.
==Group Experience Awards==
Experience points are earned through the activities of the characters, which generally relate to their adventuring goals. Thus, all characters on an adventure receive some experience points for overcoming their enemies or obstacles. Since group cooperation is important, experience points for defeating foes are given to all members of the group, regardless of their actions. Who is to say that the wizard, standing ready with a spell just in case things got ugly, might not have been necessary? Or that the bard who covered the party's escape route wasn't doing something important? A character who never hefts a sword may still have good advice or important suggestions on better tactics. Furthermore, the wizard and the bard can also learn from the actions of others.
(See also Table 31: Creature Experience Point Values and Table 32: Hit Dice Value Modifiers in the Dungeon Master Guide)
==Individual Experience Awards==
Player characters also earn experience points for individual deeds, as determined by their class. Generally, each character earns points for doing actions appropriate to his group. Warriors earn additional experience points for defeating creatures. The more difficult the battle, the greater the number of experience points. Wizards earn points for using their spells for specific purposes. The wizard who walks into the woods and casts his spells for no reason doesn't gain experience points; the wizard who casts a lightning bolt at a beholder has used his spell for a purpose. He gains experience points. Wizards also earn experience points for researching new spells and creating magical items. Priests can earn experience points for researching new spells and creating magical items. Priests can earn experience points by spreading their beliefs and using their powers in service of their deity. Rogues, who tend to have a larcenous streak, earn experience points by using their special abilities and finding or earning gold.
A character can also earn experience for the player's actions, such as playing the game well. When a player does a good job creating and pretending to be his character, the DM may give the character experience points for good role-playing. If the player is really involved and takes a major part in the game, the DM can give the player's character extra experience points. If the player uses his head to come up with a really good idea, the DM can give the character experience points for his contribution.
Finally, a character can earn experience points for successfully completing an adventure or achieving a goal the DM has set. Although a player may have a pretty good idea of what his character is supposed to accomplish, he won't know if he'll be awarded experience points for it until his character actually receives them. However, there is no rule that the DM must be consistent in these awards, or even that he must give a character anything at all.
(See also Table 33: Common Individual Awards and Table 34: Individual Class Awards in the Dungeon Master Guide)
==Training Experience==
Even when a character has earned enough experience to attain the next level, the DM may not allow immediate advancement. He may require the character to receive training to advance. When training, a character studies his skills under a tutor, taking the raw knowledge he has gained and honing it into measurable improvement. On the average, this takes a few weeks (depending on the tutor's ability), and it is normally done during the character's nonadventuring time.
A DM can also rule that the circumstances are not appropriate for the character to advance in level, such as when the game session ends with the characters deep in an abandoned mine complex. The party has just finished a battle with a band of gnolls and faces more such encounters before it can reach the surface. The DM rules that the characters receive no experience until they leave the mines, because he doesn't want them to increase in level in the middle of the adventure. He is perfectly justified in doing this. And if the characters live through the adventure, they will undoubtedly profit from it, either in experience points or knowledge gained.
==Where's the Specific Info?==
The preceding text has covered general guidelines as to how and why characters receive experience points. Since the DM actually determines how many XP each character actually receives, the detailed rules for awarding experience are given in the Dungeon Master Guide.