Block

Block is the ability of a shield to absorb melee damage in addition to its armor value. When a shield is equipped, you have a certain chance to perform a block, which reduces the damage you take by 30%. The chance to block an attack from an attacker of the same level is indicated on your character sheet under the Defense category. Fighting a mob of higher level than yourself decreases your chance to block, and vice versa.

Mob rules
While it is clear that a Blocked attack made against a player cannot be a crit, it is possible that mobs have different rules for determining whether an incoming attack is blocked or not, which includes the possibility of a "blocked crit". (c.f. Attack table)

It should be noted, however, that only special attacks&mdash;i.e. yellow-damage instant attacks such as Claw, Overpower, and Sinister Strike, and yellow-damage on-next-swing attacks such as Heroic Strike and Raptor Strike&mdash;have been observed as inflicting blocked crits. Normal white-damage auto-attacks have never been seen inflicting a blocked crit, and are probably subject to the same mutually-exclusive-attack-result rules that attacks made against players are.

Note too, that unlike player characters, mobs sometimes block attacks even though they have no shield equipped.

Chance to block
Without talents or talent-tree specialization abilities, your base chance to block an attack is 5%. This is modified by a factor based on the difference between the attacker's level and the defender's level. For each point of difference between the attacker's level and the defender's level, the block chance is modified by 0.5% if the target is a mob and 0.2% if the target is a player. Mobs level 9 and lower do not block as frequently, just as they are not missed as frequently as they should be. Also, mobs cannot block more than 5% of attacks regardless of level difference. Finally, a player's block chance is increased by block rating, and (if the character is a protection-specced warrior or paladin) by Mastery.

The formula:

Block% = 5% base chance + contribution from Block Rating + contribution from talents + contribution from Mastery (if so specced) + ((Defender's level - attacker's level) * 0.2)

Unlike Dodge and Parry, your chance to block is not subject to diminishing returns.

In combat, you will notice that the percentage of incoming attacks that are Blocked matches the Block percentage shown in your tooltips. Miss chance and Critical chance are unmodified by your Block chance, so you're not "wasting" Blocks on misses nor are you able to Block a Critical. This may seem odd to some folks if they are expecting a "if hit, then check if Parry, then check if Block, then check..." type system. WoW, like many other games, uses a combat results table-based combat scheme (where one roll determines outcome of an attack), so percentages are absolute.

Your parsed Block percentage won't necessarily match your tooltip if you're fighting creatures higher or lower in level to you. See the Defense formula above to understand.

Damage reduction
When a block occurs, the target takes the same amount of damage (s)he would have taken from an ordinary hit (not a critical hit or crushing blow), reduced by 30%. For example, if a mob would normally hit you for 500 damage, you will see in your combat log:

"... hits you for 350 (150 blocked)."

Holy Shield
The Paladin ability Holy Shield increases the amount of damage blocked by 10% when active. This increases the total damage reduction from 30% to 40%.

Critical Block
The Protection Warrior's Mastery bonus, Critical Block, gives the warrior a chance to block twice as much incoming damage. If a critical block occurs, the damage reduction is increased from 30% to 60%.