Aggro refers to gaining, controlling and losing the aggression of hostile NPCs, as well as the aggression of NPCs and any related mechanics in general. The aggro system is simple—NPCs will select targets primarily based on a range calculation, meaning the character or allied NPC such as minion or pet closest to the hostile NPC will hold its attention. However, there are additional factors such as which characters are doing damage, and how much each character has done. In addition, not all creatures (in particular, legendary creatures) use the same A.I. for aggro; some creatures will have unique aggro mechanics 
To gain the aggro of a hostile (red outlined) NPC, the player moves into the attack range of the creature. Depending on the damage and type of attack, whenever a player attacks an enemy, that player generates an amount of threat. The AI maintains an internal table that accounts for each player that attacked an enemy and a value representing the amount of threat. This table is used for several things, amongst which:
- Determining which players participated in an event or quest.
- Determining which players participated in a kill and are entitled to loot.
- Determining the next target of the hostile NPC.
As an example, a player that just mildly participates in an event (i.e. dealing damage to an NPC related to that event) and then walks away will still be stored in the aggro tables which will allow them to complete the event, albeit with low payoffs, even if they moved far away from the event area.
In the same way, using area of effect attacks to "tag" as many foes as possible during an onslaught will yield more loot than merely attacking a foe after another.
Every hostile NPC maintains an aggro table. Internally, aggro tables function on a simple priority-queue principle: the head of the queue is occupied by the player that has the most threat and the tail of the queue is occupied by the player that has accumulated the least threat. The amount of threat does not simply mean damage since there are NPCs who are attracted to a certain attribute, if a player has a high amount of toughness/vitality and wears a shield, they could hit the mob just once and have unrivaled aggro from the mob. There are also instances whereby just being in the vicinity of the mob with a high enough attribute that the NPC is attracted to, would gain aggro without participation (damaging the mob/healing a player attacking the mob). Supposing that the top player has died, disconnected, or shaken off enough aggro to no longer be at the top, the AI chooses the next player or allied NPC on the queue that has generated the most threat, and attacks them.
Gaining and losing aggro
For a neutral (yellow outlined) NPC the player must attack first to gain its attention. The aggro table of a hostile NPC changes dynamically depending on a number of factors, including:
- Closest target
- Who is dealing (or has recently dealt) damage (including pets and pending conditions)
- Random changes (if multiple players are attacking and / or within range)
- Scripted decisions (ex., some enemies such as Giganticus Lupicus will pick a specific player and always teleport to him for the duration of the fight, unless he dies)
Some players have suggested the following factors might influence certain enemies, but this is not confirmed by any official source (and seems to be based on assumptions from other MMOs that go against the principles stated by Guild Wars 2 developers):
- Top damage dealers 
- Who is using a shield / has more toughness and overall armor 
As a consequence, to lose aggro, a player should move away from the NPC and cease all attacks. This can be achieved by dodging or retreating away from the foe. Alternatively, players should turn away from the enemy and run, since backpedaling and strafing have a slower movement speed, compared to running forward.
Stealthing for long enough (or jumping onto terrain that the enemies cannot reach) can also cause them to stop attacking.
NPCs that cannot target a player nor find a path to a position where they can target the player will quickly cancel all aggro towards this player. This means that either jumping out of reach of a melee foe (NPCs cannot jump) or switching between land and water (against strictly aquatic or terrestrial NPCs):
- Is effective in breaking aggro;
- Is not effective in killing it safely, especially if there is no other ally to keep the NPC in combat mode.
An NPC who loses aggro towards everything usually turns invulnerable for a short while, long enough to regenerate entirely, and proceeds slowly to its spawning area. However, once at full health, most of the time it becomes available again to attack even if it is still moving.
Although Guild Wars 2 does not allow players to become unkillable tanks like in other MMOs (mainly because of the lack of effective long-term healing), keeping the aggro of a NPC while kiting, blocking or otherwise dodging their attacks remains a valid strategy. However, different NPCs use different aggro mechanics and widely varying types of attacks, and a defensive tactic that is useful against some NPCs can utterly fail against others.
- One tactic that involves manipulating the aggro table is called kiting. This usually means to generate a large amount of threat for a hostile NPC and then run, avoiding the NPC's attacks so that other party members may kill the NPC without taking any damage. Being under effect of swiftness greatly helps kiting.
- Another tactic involves reviving fallen allies in turns to draw a NPC across the battlefield, while the other players attack it. Enemies value attacking a player character performing a revive on a downed ally,  to the point of sometimes ignoring any other factor, while stopping the revive will then draw their attention back to the classical threat sources.