Strength (often abbreviated STR) has the following effects:

•Increases attack power with melee weapons:
-Warriors, death knights, paladins, mages, priests and warlocks gain 2 melee attack power per point of strength.
-Druids gain 1 melee attack power per point of strength.
•Increases parry rating for warriors, paladins, and death knights by 27% of strength.
•Paladin talent Touched by the Light (Protection, Spec passive) increases spell power by 60% of Strength.

Strength does not affect the chance to land a critical strike. Strength does not improve the chance to block.

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Agility (often abbreviated AGI) has the following effects:

•Increases attack power with ranged weapons (not including wands) or melee weapons for certain classes.
-Warriors and rogues gain 1 ranged attack power for each point of agility.
-Hunters gain 2 ranged attack power for each point of agility.
-Rogues, shamans and druids in Cat Form and Bear Form gain 2 melee attack power per point of agility.
-Hunters gain 1 melee attack power per point of agility.
•Increases chance to score a critical hit with a weapon.
•Increases chance to dodge attacks. This is dependent on both class and level, in fact death knights, paladins and warriors don't.

Agility Equivalence Points is a notable formula for calculating the contribution of agility. It might be outdated with regard to hunters.

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Intellect (often abbreviated as INT) has the following effects:

•Increases spell power by 1 per point at level 80 or above. Intellect is now the main source of spell power for most cases.
•Increases mana pool. Each point of intellect gives player characters 15 mana points (except from the first 20 points of Intellect that provide 1 mana for each point instead). Warlock minions gain 1 mana per point of base intellect, imps gain ~4.9 mana for each additional point of intellect, while all other minions gain ~11.5 mana for each additional point of intellect.
•Increases chance to score a spell critical strike. This increase varies by class and level; in general, the relationship is linear as intellect increases. The rate of increase is generally based on the amount of intellect a class is expected to have at a certain level.

With Mists Of Pandaria, Intellect will no longer affect (increase) the mana pool.

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Spirit (often abbreviated SPI) has the following effects:

•Increases in-combat mana regeneration for healers. It's the primary attribute for mana regeneration of healers.
•Adds 1 spell hit rating per point for shadow priests, elemental shaman and balance druids.[citation needed]
-Paladins who pick up the talent Enlightened Judgements gain spell hit equal to 50% spirit (Rank 1) or 100% spirit (Rank 2)

Spirit is of no use for mages and warlocks due to how their classes' mana regeneration works.
Spirit is made more valuable by abilities that increase other stats based on Spirit.