Constitution is one of the six character abilities. It adds a bonus to hitpoints for the first 9 nine levels for all warrior classes and the first 10 levels of all other class groups. For example, if a character's constitution is permanently boosted by one point and the character is a level 11 warrior, bonus hit points for levels 1-9 are added retroactively.
Warrior classes, with the exception of the Monk, gain bonus hitpoints from a Constitution of 17 or higher, while non-warrior classes gain a maximum of +2 hitpoints at 16 Constitution. This character ability is essential for all classes, but more so for fighters, paladins, rangers and barbarians. Paladins and rangers have a high minimal requirement for constitution.
Constitution scores of less than 7 incur a penalty to your hitpoints, to a minimum of +0 to your maximum health at level up (i.e. no change). A constitution score between 7 and 14 gives no modifier to HP adjustments.
Characters with more than 20 or a higher constitution, will also regenerate lost hitpoints during and outside of battles. This regeneration affects all classes.
Gnomes, Dwarves, and Halflings get bonuses to their saving throws based on their constitution, up to a maximum of +5 at 18 constitution. Collectively known as the "shorty bonuses", these bonuses are re-calcuated on level-up. Gnomes receive this bonus to their Wands and Spells saving throws, while Dwarves and Halflings both have the bonus added to their Death, Wands and Spells saving throws.
(Fighter, Paladin, Ranger, Barbarian)
Saving Throw Bonus
(Gnome, Dwarf, Halfling)