Constitution represents the health and stamina of a character. High constitution increases the number of hit points a character has (affecting how much damage they can take), and this makes it important for everyone, but especially so for fighters. If constitution ever increases, hit points increase retroactively as well. Spellcasters also need strong constitution to keep their spells from being interrupted during combat.
Specials: The constitution modifier is added to the hit points gained each character level, subject to a minimum of 1 hit point per level.
Constitution is unusual among the abilities in that, while it is useful for all characters, few characters benefit enough from exceptional constitution to make it worth the cost. Consequentially, few players will leave constitution as low as possible, but at the same time, few choose constitution when a base ability score can be increased (every four levels). Among those who do choose to increase the base constitution score, most are satisfied upon reaching a base score of 22, the first increase in the constitution modifier after qualifying for epic damage reduction.
The addition to hit points is dynamic in that whenever a character's constitution changes (up or down), the hit point bonus (or penalty) is recalculated, changing that character's maximum and current hit point levels. Bonus hit points from constitution are added "underneath" a character's rolled hit points. That is, damage is taken first from the rolled hit points, then from the bonus from constitution. The result is that the difference between maximum and current hit points is retained. Thus, a creature with few hit points may be killed by a reduction in constitution. For example, if a level 4 character has been reduced to 3 hit points, then reducing that character's constitution modifier by 1 will immediately remove 4 hit points from the character's maximum and current hit points, leaving the character dying with -1 hit points (unless the above minimum comes into play). In a situation like this, experience points (if any) are awarded to the last character to injure the victim, not necessarily the one who caused the reduction in constitution.