As player characters overcome challenges, they gain experience points. As these points accumulate, PCs advance in level and power. The rate of this advancement depends on the type of game that your group wants to play. Some prefer a fast-paced game, where characters gain levels every few sessions, while others prefer a game where advancement occurs less frequently. In the end, it is up to your group to decide what rate fits you best. Characters advance in level according to Table: Character Advancement and Level-Dependent Bonuses.
A character advances in level as soon as he earns enough experience points to do so—typically, this occurs at the end of a game session, when your GM hands out that session's experience point awards.
The process of advancing a character works in much the same way as generating a character, except that your ability scores, race, and previous choices concerning class, skills, and feats cannot be changed. Adding a level generally gives you new abilities, additional skill points to spend, more hit points, and possibly an ability score increase or additional feat (see Table: Character Advancement and Level-Dependent Bonuses). Over time, as your character rises to higher levels, he becomes a truly powerful force in the game world, capable of ruling nations or bringing them to their knees.
When adding new levels of an existing class or adding levels of a new class (see Multiclassing, below), make sure to take the following steps in order. First, select your new class level. You must be able to qualify for this level before any of the following adjustments are made. Second, apply any ability score increases due to gaining a level. Third, integrate all of the level's class abilities and then roll for additional hit points. Finally, add new skills and feats. For more information on when you gain new feats and ability score increases, see Table: Character Advancement and Level-Dependent Bonuses.
Instead of gaining the abilities granted by the next level in your character's current class, he can instead gain the 1st-level abilities of a new class, adding all of those abilities to his existing ones. This is known as “multiclassing.”
For example, let's say a 5th-level fighter decides to dabble in the arcane arts, and adds one level of wizard when he advances to 6th level. Such a character would have the powers and abilities of both a 5th-level fighter and a 1st-level wizard, but would still be considered a 6th-level character. (His class levels would be 5th and 1st, but his total character level is 6th.) He keeps all of his bonus feats gained from 5 levels of fighter, but can now also cast 1st-level spells and picks an arcane school. He adds all of the hit points, base attack bonuses, and saving throw bonuses from a 1st-level wizard on top of those gained from being a 5th-level fighter.
Note that there are a number of effects and prerequisites that rely on a character's level or Hit Dice. Such effects are always based on the total number of levels or Hit Dice a character possesses, not just those from one class. The exception to this is class abilities, most of which are based on the total number of class levels that a character possesses of that particular class.
Each character begins play with a single favored class of his choosing—typically, this is the same class as the one he chooses at 1st level. Whenever a character gains a level in his favored class, he receives either + 1 hit point or + 1 skill rank. The choice of favored class cannot be changed once the character is created, and the choice of gaining a hit point or a skill rank each time a character gains a level (including his first level) cannot be changed once made for a particular level. Prestige classes (see Prestige Classes) can never be a favored class.
Source: Advanced Player's Guide
The normal benefit of having a favored class is simple and effective: your character gains one extra hit point or one extra skill rank each time she gains a level in that class (or in either of two classes, if she is a half-elf*). The alternate favored class abilities listed here may not have as broad an appeal as the standard choices. They are designed to reflect flavorful options that might be less useful in general but prove handy in the right situations or for a character with the right focus. Most of them play off racial archetypes, like a half-orc’s toughness and proclivity for breaking things or elven grace and finesse.
In most cases, these benefits are gained on a level-by level basis—your character gains the specified incremental benefit each time she gains a level. Unless otherwise noted, these benefits always stack with themselves. For example, a human with paladin as a favored class may choose to gain 1 point of energy resistance each time she gains a level; choosing this benefit twice increases this resistance bonus to 2 per level, 10 times raises it to 10 per level, and so on.
In some cases this benefit may eventually hit a fixed numerical limit, after which selecting that favored class benefit has no effect. Of course, you can still select the bonus hit point or skill rank as your favored class benefit, so there is always a reward for sticking with a favored class.
Finally, some of these alternate favored class benefits only add +1/2, +1/3, +1/4, or +1/6 to a roll (rather than +1) each time the benefit is selected; when applying this result to the die roll, round down (minimum 0). For example, a dwarf with Rogue as his favored class adds +1/2 to his trap sense ability regarding stone traps each time he selects the alternate Rogue favored class benefit; though this means the net effect is +0 after selecting it once (because +1/2 rounds down to +0), after 20 levels this benefit gives the dwarf a +10 bonus to his trap sense (in addition to the base value from being a 20th-level Rogue).
Each race page includes a set of alternative benefits that characters of that race may choose instead of the normal benefits for their favored class. Thus, rather than taking an extra hit point or an extra skill rank, players may choose for their characters to gain the benefit listed here. This is not a permanent or irrevocable choice; just as characters could alternate between taking skill ranks and hit points when they gain levels in their favored class, these benefits provide a third option, and characters may freely alternate between them.
As with any alternate or optional rule, consult with your GM to determine whether exchanging normal favored class benefits for those in this chapter will be allowed.
Although classes doesn't describe what happens after 20th level, this isn't to say that there are no resources available to you should you wish to continue your campaign on to 21st level and beyond. Rules for epic-level play like this exist in numerous products that are compatible with the Pathfinder Roleplaying Game, although in many cases these alternative rules can provide unanticipated problems. For example, if your campaign world is populated by creatures and villains who, at the upper limit of power, can challenge a 20th-level character, where will epic-level PCs go for challenges? You might be looking at creating an entirely new campaign setting, one set on different planes, planets, or dimensions from the one where your players spent their first 20 levels, and that's a lot of work.
Paizo Publishing may eventually publish rules to take your game into these epic realms, but if you can't wait and would rather not use existing open content rules for epic-level play, you can use the following brief guidelines to continue beyond 20th level. Note that these guidelines aren't robust enough to keep the game vibrant and interesting on their own for much longer past 20th level, but they should do in a pinch for a campaign that needs, say, 22 or 23 experience levels to wrap up. Likewise, you can use these rules to create super-powerful NPCs for 20th-level characters to face.
To gain a level beyond 20th, a character must double the experience points needed to achieve the previous level. Thus, assuming the medium XP progression, a 20th-level character needs 2,100,000 XP to become 21st level, since he needed 1,050,000 XP to reach 20th level from 19th. He'd then need 4,200,000 XP to reach 22nd level, 8,400,000 XP to reach 23rd, and so on.
Hit dice, base attack bonuses, and saving throws continue to increase at the same rate beyond 20th level, as appropriate for the class in question. Note that no character can have more than 4 attacks based on its base attack bonus. Note also that, before long, the difference between good saving throws and poor saving throws becomes awkwardly large—the further you get from 20th level, the more noticeable this difference grows, and for high-level characters, bolstering their poor saving throws should become increasingly important. Class abilities that have a set, increasing rate, such as a barbarian's damage reduction, a fighter's bonus feats and weapon training, a paladin's smite evil, or a rogue's sneak attack continue to progress at the appropriate rate.
A spellcaster's caster level continues to increase by one for each level beyond 20th level. Every odd-numbered level, a spellcaster gains access to a new level of spell one above his previous maximum level, gaining one spell slot in that new level. These spell slots can be used to prepare or cast spells adjusted by metamagic feats or any known spell of lower levels. Every even-numbered level, a spellcaster gains additional spell slots equal to the highest level spell he can currently cast. He can split these new slots any way he wants among the slots he currently has access to.
For example, a 21st-level wizard gains a single 10th-level spell slot, in which he can prepare any spell of level 1st through 9th, or in which he can prepare a metamagic spell that results in an effective spell level of 10 (such as extended summon monster IX, or quickened disintegrate). At 22nd level he gains 10 spell-levels' worth of new spell slots, and can gain 10 1st-level spells per day, two 5th-level spells per day, one 7th-level and one 3rd-level spell per day, or one more 10th-level spell per day. At 23rd level, he gains a single 11th-level spell slot, and so on.
Spellcasters who have a limited number of spells known (such as bards and sorcerers) can opt out of the benefits they gain (either a new level of spells or a number of spell slots) for that level and in exchange learn two more spells of any level they can currently cast.
You might want to further adjust the rate of spell level gain for classes (like paladins and rangers) who gain spells more slowly than more dedicated spellcaster classes.
The simplest way to progress beyond 20th level is to simply multiclass or take levels in a prestige class, in which case you gain all of the abilities of the new class level normally. This effectively treats 20th level as a hard limit for class level, but not as a hard limit for total character level.