Evasion is a rating that measures a character's ability to avoid being struck by enemy attacks. Evasion is opposed by the attacker's Accuracy.
All classes start with 53 base evasion rating and gain 3 additional evasion per level. This gives a base of 350 at level 100 (not considering dexterity).
Dexterity also grants bonuses to evasion. Every 5 points of dexterity provides 1% increased evasion rating.
Movement speed is beneficial for characters that employ evasion as the primary form of damage reduction, since it requires a play style where physically avoiding enemy spells and attacks is often necessary.
Mechanics[]
Evading an attack prevents all damage and other harmful effects including status ailments and stun. Only melee and ranged attacks can be evaded; spells cannot. The chance to evade an attack is based on evasion verses the attacker's accuracy. Chance to evade can never be lower than 5%, nor higher than 95%.
Chance to Evade = 1  Attacker's Accuracy / ( Attacker's Accuracy + (Defender's Evasion / 4) ^ 0.8 )
Evasion also gives an implicit chance to avoid taking critical strikes from attacks. Critical strikes from spells can't be evaded. If an incoming critical strike succeeds its hit roll, a second hit roll is performed to determine if the critical is evaded or not. If this roll succeeds, a critical strike is scored. If it fails, the attack still hits, but only for regular damage.
Evasion is not actually based on chance in the technical sense, but can still be referred to as such. The details have now been posted by GGG_Mark [[1]] in the mechanics thread. Essentially each entity has an 'entropy'value used to eliminate streaks. To paraphrase his post, these are the steps that occur for the evasion check:
 If the time between the last accuracyevasion check and this one is larger than timegap t, randomise the entropy from 099. (This gap is small but not known to us.)
 Calculate chance to hit of the attacker, and add this to the entropy counter.
 If this is 100 or greater, the check counts as a hit. Subtract 100 from the entropy value.
 A critical hit is evaded on a separate random roll and will not affect this entropy value.
This system is designed to evenly spread out hits and misses such that players always evade an average number of times according to their ‘chance’ to evade. For example, if a set of identical monsters attacks a particular player and have 25% chance to hit him/her, only 1 in every 4 attacks will connect. This will hold true until said gap has passed. Note that the chance to evade calculation on the character page is based on the average accuracy of a monster at the player's level.
If one would like to prove this, a good test is to take an evasion character with 40+% evade with Arrow Dodging to a bunch of act 2 spitting cobras, and sit with a bunch of potions. No matter how many normal cobras you gather, you should be able to count roughly an equal the number of misses between each hit and never get two hits in a row.
Interaction with Dodge[]
Chance to Dodge bonuses granted by sources like Acrobatics function similar to evasion, but are rolled separately and are pseudorandom rather than entropybased. Phase Acrobatics grants a chance to dodge spells.
Assuming a character has a 40% chance to evade and a 20% chance to dodge, a character has a 52% chance to avoid taking damage.
Chance to Avoid Attack = 1  ( (1  Chance to Evade) * (1  Chance to Dodge) )
Passives[]
There are passive skill nodes that grant additional evasion or a combination of bonuses including evasion.
Evasion[]
These passives increase evasion rating by a percentage.
In the case of Arrow Dodging, the chance to evade projectile attacks is doubled.
Icon  Name  Bonuses 

File:Passiveevasion.png  Evasion 

File:Passivereflexes.png  Reflexes 

Evasion and Energy Shield 
 
Nullification 
 
File:Passivearmourandevasion.png  Armour and Evasion 

File:Passiveleatherandsteel.png  Leather and Steel 

File:Passivefrenzychargesmall.png  Evasion per Frenzy Charge 

Arrow Dodging 

Miscellany[]
These passives are related to evasion, but don't fit into any other category.
Icon  Name  Bonuses 

Iron Reflexes 
 
Unwavering Stance 
