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Constitution (Con) represents a character's health, stamina, and vital force. Because Constitution affects hit points and healing surges, it is a benefit to all player characters.

Constitution is the key ability for Endurance skill checks.

A character's Constitution score (not modifier) contributes to his or her maximum number of hit points.

A character's Constitution modifier contributes to his or her number of healing surges per day.

In addition, a character's Constitution modifier or Strength modifier, whichever is higher, will contribute to Fortitude defense.

Constitution and Strength are redundant with each other for the purpose of Fortitude defense. For this reason, Constitution may be less useful for characters who need Strength for attack rolls, class features, or skill checks.

Classes with Constitution as a primary key ability
warlock (Infernal, Star, or Vestige Pact original warlock)
Classes with Constitution as a secondary key ability
ardent (Cha primary, Mantle of Elation or Mantle of Impulsiveness)
artificer (Int primary, healing infusion: resistive formula)
assassin (Dex primary)
barbarian (Str primary, Rageblood Vigor or Thaneborn Wrath original barbarian)
bard (Cha primary, Virtue of Valor original bard)
cleric (Wis primary, warpriest)
druid (Wis primary, Primal Guardian or Primal Swarm original druid, or sentinel, or Circle of Renewal protector)
fighter (Str primary, Battlerager Vigor)
invoker (Wis primary, Circle of Malediction or Circle of Wrath)
monk (Dex primary, Iron Soul)
runepriest (Str primary, Wrathful Hammer)
shaman (Wis primary, Protector Spirit or World Speaker Spirit)
sorcerer (Cha primary, Wild Magic)
swordmage (Int primary, Aegis of Shielding)
warden (Str primary, Earthstrength)
wizard (Int primary, Staff of Defense or Tome of Binding arcanist, Evoker or Pyromancer mage)
Races with a bonus to Constitution
dwarf, goliath, minotaur, mul, orc, warforged (Con/Str)
bullywug, gnoll, halfling, half-orc, kobold, revenant (Con/Dex)
genasi, githyanki, hobgoblin, warforged (Con/Int)
duergar, dwarf, half-elf, mul, svirfneblin, wilden (Con/Wis)
dragonborn, half-elf, hobgoblin, kobold, satyr, tiefling (Con/Cha)