Aggro refers to gaining, controlling and losing the aggression of hostile NPCs. The aggro system is simple—NPCs will select targets primarily based on a range calculation, meaning the character or pet closest to the 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 use the same AI 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 (ie: 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.
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 dealt the most threat and the tail of the queue is occupied by the player that has accumulated the least threat. Supposing that the top player has died, disconnected, or shaken off aggro, the AI pops the next player or allied NPC off 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, in order of importance  :
- closest target to them
- who is dealing damage
- top damage dealers
- who is using a shield / has more toughness and overall armor
- others (see Tanking tactics below)
As a consequence, to lose aggro, the player moves to be further from the NPC than other players or allies and ceases all attacks. A typically effective Guild Wars 2 technique to lose aggro is to roll away (dodge) from the hostile NPC. This is based on the fact that the distance travelled away from the hostile NPC by rolling is greater than the distance a player could run in the same amount of time. Lastly, a jump seems slower than a run - jumping away from the NPC will not help on a flat surface.
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.
Indeed, a 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 his attacks remains a valid strategy, if a difficult one, especially against melee foes. However, keep in mind that different NPCs use different aggro mechanics, and that a tactic that is useful against some NPCs can utterly fail against another.
- 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.
- Another tactic involves reviving fallen allies in turns to draw a NPC accross the battlefield, while the other players attack it. Highly challenging enemies such as Champions and event bosses indeed value reviving to the point of sometimes ignoring any other factor, while stopping reviving will draw their attention to the more classical threat sources.