Use your favourite features in-game with our Desktop AppEnergy Shield is a defensive resource that protects Life by default. Any Damage that would be dealt to Life is dealt to Energy Shield first, with the exception of Damage from Bleeding or Poison.
When Chaos Damage is dealt to Energy Shield it removes twice as much, except for any Chaos Damage coming from Poison which instead directly bypasses Energy Shield to deal Damage to Life.
A Hit that would inflict Bleeding must deal some or all of its Damage to Life to be able to inflict the Ailment. Therefore a character with sufficient Energy Shield is pseudo-immune to Bleeding as their Energy Shield would need to be entirely depleted before a monster is able to inflict Bleeding on them. This also means that monsters need to have their Energy Shield broken before a character can inflict Bleeding on them. However, Damage from Bleeding will directly bypass Energy Shield to deal Damage to Life.
Energy Shield has an inherent Recovery mechanic known as Recharge. By default Recharge recovers 12.5% of Energy Shield per second and begins after a period of 4 seconds where no Energy Shield is lost through taking Damage. Taking Damage to Energy Shield during Recharge will interrupt the Recharge and you'll need to wait out the delay before it starts again.
Note: Recharge is a form of Recovery and is not considered to be Regeneration.
You can utilise modifiers that improve Recharge, such as increasing the amount of recovery or shortening the delay before Recharge begins. 
Rapid Recharge is one such example of these types of modifiers.
Similar to other modifiers in Path of Exile 2, 'faster start of Energy Shield Recharge' does not apply a direct reduction to the delay timer, and instead alters the effective time that elapses. For example, with '100% faster start of Energy Shield Recharge', a character's Recharge would begin after a period of 2 seconds without taking Damage to Energy Shield.
There are also some mechanics that can force Energy Shield to immediately start Recharging and others that will stop Recharge from being interrupted by taking Damage. The Energy Barrier Support is a good example of both.
Energy Shield is primarily gained from gear: Helmets, Body Armours, Gloves, Boots and Foci. The majority of the Intelligence-aligned base types have inherent Energy Shield present as the Defence stat that's tied to the item. This portion of Energy Shield can be scaled by local modifiers, for example 'X% increased Energy Shield' or '+X to maximum Energy Shield'. These local modifiers scale the Energy Shield gained from the piece of gear directly and do not apply globally to your character. The Quality stat is a multiplier on the final amount of Energy Shield on a piece of gear.
A local modifier is one that only applies to a stat on the item it's present on, whilst a global modifier will apply to the total stat on the character. Local modifiers are found on items with inherent stats, for example Gloves with inherent Armour will be able to roll local modifiers that improve the inherent Armour on the item. However on items where there is no inherent stat such as Jewels, or on items that can roll modifiers that do not apply to the inherent stat, such modifiers are typically global modifiers.
For example the Amulet below has a modifier that grants '38% Increased Energy Shield' which is applied globally to the character's flat Energy Shield, as it cannot apply locally to any stat on the item.
The Helmet below has'133% Increased Energy Shield', which is a local modifier that's not applied globally to the stat on your character. Instead it scales the inherent Energy Shield on the item, making it grant a higher amount of flat Energy Shield.
Flat Energy Shield gained from equipment or other sources is added together and scaled by any global Energy Shield modifiers you have, such as 'X% increased maximum Energy Shield' from the passive skill tree.
Note: modifiers to Global Defences will also scale Energy Shield.
Energy Shield can also be obtained from other gear slots such as Rings, Amulets and Belts, from rollable Prefix modifiers. There are also some Unique items that grant special modifiers, such as 
Atziri's Disdain which provides '25-30% of Maximum Life as Extra Maximum Energy Shield', but also makes a portion of Damage directly bypass Energy Shield to deal Damage to Life.
The 
Chaos Inoculation Keystone, known as CI, makes a character Immune to Chaos Damage whilst setting their maximum Life to 1. This Keystone is most often utilised to make Energy Shield function as a character's primary hit pool, as bypassing Damage from Poison no longer deals any Damage due to the character being Immune to Chaos Damage, whilst Bleeding cannot be inflicted on them unless their Energy Shield is entirely depleted.
Note: when Chaos Inoculation is allocated, a character's Life globe is transformed to display Energy Shield instead.
The 
Eternal Youth Keystone makes Life Recharge instead of Energy Shield, whilst the 
Tetzlapokal's Desire Unique Body Armour simply allows Life to also Recharge without removing the mechanic from Energy Shield.
Life Recharge inherits the mechanics of Energy Shield Recharge but applies them to Life instead. For example, Recharge will be interrupted upon taking Damage to Life and will only begin after a default period of 4 seconds where no Damage is dealt to Life.
Life Recharge is affected by all modifiers to Energy Shield Recharge, such as 'faster start of Energy Shield Recharge' or 'increased Energy Shield Recharge rate'. However, modifiers that would improve Energy Shield Recovery specifically such as 'increased Energy Shield recovery rate' will not apply to Life Recharge. Instead, modifiers that improve Life Recovery such as 'increased Life Recovery rate' do apply to the Recovery gained from Life Recharge.
The 
Eldritch Battery Keystone converts 100% of Energy Shield into Mana. Only flat Energy Shield is converted before any Energy Shield scaling modifiers are applied, and the resulting Mana can only be scaled with modifiers that apply to Mana.
