Dexterity

Dexterity is a Character Attribute present in the Diablo universe.

Diablo I/Hellfire
Dexterity is represented in its most basic and limited form in Diablo and Hellfire. It contributes to a hero's To-Hit %, and Armor Class. In addition, as a hero's Dexterity grows, certain swords and all bows begin to open themselves for use. † Ingame, this blocking bonus is bugged. After the first game, the hero loses his blocking bonus, and can only rely on his Dexterity and clvl to do the work for his blocking chance.
 * Dexterity contributes to To-Hit % at the rate of 1% per 2 points.
 * Dexterity contributes to Armor Class at the rate of 1 per 5 points.
 * Dexterity contributes to a hero's Chance to Block % in the following formula: Dexterity + (2 * clvl) + bonus†, subtract a number equal to the attacker's level * 2. There is no 'hard cap', meaning that 100% block chance is legitimately possible.

In addition, the Rogue, Monk, and Bard all rely on Dexterity for damage, although they still accept a bonus from Strength as well: Unfortunately, due to how high the classes' caps on Dexterity are, only the Rogue stands a chance at hitting targets with only items that give Dexterity. All other classes need affixes that give To-Hit % chance
 * Rogue in all cases = (Strength + Dexterity) * clvl/200
 * Bard with a sword, Monk unarmed or using a staff = (Strength + Dexterity) * clvl/150
 * Monk in all other cases = (Strength + Dexterity) * clvl/300

Diablo II
Dexterity affects Attack Rating and Defense Rating.

Some melee weapon damage, such as javelins, daggers, claws, and all Ranged Weapons damage increase with Dexterity. Dexterity is also required to use some weapons, such as bows and swords. However, it is not generally required for some weapons, such as maces and axes.

Dexterity also increases chance to block. The chance to block is capped at 75 percent.

Diablo III
As of Patch 2.1, Dexterity increases Armor (1 Armor for 1 Dexterity), in the same way Strength does for all classes. Monks and Demon Hunters (both use dexterity as their primary attribute) also increase all damage output by 1% for each point in Dexterity.

Monks and Demon Hunters gain 3 Dexterity per level, while other classes gain 1, and may receive 5 points per Paragon point invested in it. Dexterity can also be gained by placing Emerald gems into empty armor sockets.

Development
Originally, each point of dexterity was increasing the player's chance to dodge, regardless of class. The gains were big at first, but they would also suffer from diminishing returns, not recommended to classes with other primary attributes. In patch 2.1., developers wanted to replace Dodge bonus with Armor, in order to make this bonus more predictable and reliable.