|d20 Modern System Reference Document
The GM can improve a creature by increasing its Hit Dice. The Advancement entry indicates the increased Hit Dice (and often size) of the creature or indicates that the creature can advance by character class.
Increasing Hit Dice
As a creature gains Hit Dice, many of its game statistics change.
Size: Adding Hit Dice to a creature can also increase its size. An increase in size affects a creature’s Defense, attack rolls, and grapple checks, as shown on Table: Creature Sizes, as well as physical ability scores and damage, as shown in the descriptions of the creature types.
Defense: An increase in size affects a creature’s Defense, as shown on Table: Creature Sizes. An increase in size might also improve a creature’s natural armor bonus to Defense, as shown on Table: Adjustments to Physical Abilities and Natural Armor. Note that a natural armor bonus stacks with an equipment bonus from armor.
Attack Bonus: Table: Creature Saves and Base Attack Bonuses shows how a creature’s base attack bonus improves as it gains Hit Dice. A change in the creature’s size also modifies its attack rolls, as shown on Table: Creature Sizes. Both values must be counted when recalculating a creature’s attack bonus.
Grapple Modifier: An increase in size affects a creature’s grapple modifier, as shown on Table: Creature Sizes.
Damage: An increase in size also increases the amount of damage a creature deals with its natural weapons.
Saving Throws: Table: Creature Saves and Base Attack Bonuses shows how a creature’s saving throw bonuses improve as it gains Hit Dice.
Ability Scores: An increase in size affects a creature’s Strength, Dexterity, and Constitution, as shown on Table: Adjustments to Physical Abilities and Natural Armor.
Skills and Feats: As shown on Table: Bonus Skill Points and Feats by Creature Type, a creature may gain additional skill points and feats depending on its type.
Acquiring a Character Class
A creature that acquires a character class follows the rules for multiclass characters. The creature’s character level equals its Hit Dice plus the number of character class levels it has. Creatures with 1 or fewer Hit Dice count only their character class levels.
Size: Adding character classes to a creature never affects its size.
Skills: Creatures that take levels of a character class do not gain as many skill points as a human character of the same class. Creatures get 4 fewer skill points at 1st level than a human character and 1 fewer skill point each level thereafter; see Table: Skill Points per Class Level for Nonhumans.
Feats: Creatures with 1 or fewer Hit Dice that acquire character class levels advance as human characters do, but they gain only one bonus feat at 1st level instead of two.
Talents and Abilities: Creatures that advance by character class gain special talents and abilities, which are noted here.