All characters who survive an adventure and return to safety gain Experience points (XP) from monetary treasure recovered and monsters defeated. XP is awarded by the referee. Awarding XP explains how to do this.

Characters with especially low or high scores in their class’s prime requisites receive a bonus or penalty to XP earned (see Ability Scores). These modifiers are applied to the total XP gained by the character following an adventure.

When a character has accumulated enough XP to reach the next Experience level, the player should consult the description of the character’s class and note any improvements in saving throws, attack probabilities, spells per day, and other class abilities.

Up to 9th level: The character gains a Hit Die. The player should roll the Hit Die type for the character’s class, apply the character’s CON modifier, and add the result to the character’s current and maximum hit point totals.
10th level and higher: The character’s hit points increase by a flat amount listed in the class progression table. No Hit Dice are gained.

Characters cannot advance more than one level in one session. Any additional XP that would take a character two or more levels above their current level are lost, leaving the character at 1 XP below the total for the next level.