Expertise

Expertise (also known as Exp and Ept) was a secondary attribute introduced in Patch 2.3.0. Each percent of expertise decreased the chance that melee or ranged attacks made by the player will be dodged by the same percentage. If the dodge chance was completely negated by expertise, any leftover expertise would then negate parried by the same amount. With Patch 5.0.4, Expertise also increased Spell hit for casters by the same amount.

For example, a special attack against a boss level target would have 7.5% to be dodged and 7.5% chance to be parried (from the front only).
 * 6.5% expertise would reduce the dodge chance by 6.5% (to 1%), would not affect the parry chance and will add 6.5% to spell hit.
 * 7.5% expertise would eliminate the dodge chance completely, would not affect the parry chance and will add 7.5% to spell hit.
 * 8.5% expertise would eliminate the dodge chance completely, would reduce the parry chance by 1% (to 6.5%) and will add 8.5% to spell hit.

Expertise was removed with Patch 6.0.2 along with hit rating. The developers described the process of reaching the expertise and hit caps as a penalty rather than an augmentation, with players inevitably regarding the stats as mandatory rather than "fun". As a result, the stats were removed, with players effectively automatically capped in both expertise and hit.

Expertise rating
Note that there is a difference between "expertise" and "expertise rating". Expertise is the attribute itself; expertise rating is a rating which is converted into expertise at a ratio determined by your level. For example, a level 90 character will gain 1% expertise for every 340 expertise rating. Expertise rating is the main source for expertise, typically gained from gear and enhancements such as gems and enchantments.

Racial bonuses
Certain race's racial traits provide passive expertise bonuses. All racials provide a 1% bonus to expertise for certain types of weapons.

These bonuses are applied to weapons individually. An orc dual-wielding an axe and a mace would therefore benefit from a 1% expertise bonus to the axe, but not to the mace. The 1% expertise granted by these racials is equivalent to 340 rating at level 90.

Expertise caps
For boss-level mobs (which are calculated as level + 3) the soft cap is 7.5% dodge or 7.5% expertise (2550 expertise rating), and the hard cap is an additional 7.5% parry or 15% expertise (5100 expertise rating at level 90). Melee DPS classes will usually focus on the soft cap, since bosses cannot parry when attacked from behind. Some tanks will like to collect a lot of expertise in order to avoid boss parries, which may result in the Parry-Hasting effect. However, recently the boss Parry-Hasting effect has been disabled on some of the bosses.

For level 87 and below (heroic bosses and trash mobs in raids) the dodge and parry caps are the same.

Tanking
Expertise is of two-fold use to tanks. First, by reducing the target's avoidance, it improves a tank's threat generation (more of his/her attacks will land). More importantly, because mobs may attack more quickly after parrying an attack, expertise reduces the total damage that the tank may receive by reducing the chance that his target parries. The increase in attack speed varies (from 0-40% of the speed of the parried attack), and can stack if multiple attacks are parried in rapid succession, leading to a potential source of massive burst damage. However, this parry-hasting effect from when a boss parries has been disabled from most bosses except the last 3 in 25m TOC.
 * Note – The numbers here indicate the numbers necessary to stop your target from parrying and dodging your attacks, which apply primarily to tanks (due to tanks attacking the target from the front, and thus within the arc of parry-possible attacks). For dodge-only caps, see DPS Expertise caps, below.
 * The expertise rating needed to achieve each expertise percentage at level 90 is provided beside each number in parentheses. These numbers are rounded up, and the formula used is 1% expertise = 340 expertise rating.

DPS
Melee DPS also benefit from expertise. With sufficient expertise, melee classes can attack their target from the front without being dodged or parried (although facing restrictions, such as those on Backstab or Shred, still apply). It is possible to have enough expertise to prevent dodges and parries, but parries are much harder to prevent, and players without such high expertise values should remain behind the target to prevent parried attacks from increasing the target's attack speed against the tank.

DPS will need a total of 7.5%, or 2550 expertise rating, to avoid Dodges on yellow attacks against a raid boss.
 * Note that DPS expertise scores are assuming that the player is attacking the target from behind, and thus only take dodged attacks into account. For targets that can parry attacks (i.e. from with tanks, standing in front of the target), the expertise ratings required to make both parries and dodges impossible would be much higher (see Tanking Expertise Caps, above).
 * The expertise rating needed to achieve each expertise percentage at level 90 is provided beside each number in parentheses. These numbers are rounded up, and the formula used is 1% expertise = 340 expertise rating.

Expertise against bosses
The dodge and parry chances for boss-mobs is 7.5% dodge and 7.5% parry. This means that a player would need 7.5% expertise (2550 expertise rating) to negate a boss's dodge chance, and 15% expertise (5100 rating) to prevent parries.

To "cap" expertise against a raid boss, melee DPS need only concern themselves with the boss' dodge chance, provided that they attack from behind. This dictates a goal of 2550 expertise rating. Tanks, however, attack from the front, and need to worry about both dodge and parry.