Aggro, short for aggravate, is an internal measurement for how much of a threat the AI considers an individual player to be. The more aggro a player holds, the more highly that player will be prioritized for attacks and skill usage by the AI. Understanding how to gain, hold, and break aggro is called aggro control or aggro management.
For most of the high-end PvE content such as The Underworld and Hard Mode, each party member must have a solid understanding of aggro management in order for the party to succeed, because a single player's mistake can turn a potential victory into a near-certain wipe.
Tanking is a popular form of aggro management which, when done successfully, makes both keeping the party alive and killing foes quicker and easier than if the whole group held aggro.
 Gaining aggro
Gaining aggro or holding aggro means increasing an ally's threat level, therefore making that ally more likely to be attacked by the AI.
 General PvE techniques
- Enter a foe's danger zone.
- Within the danger zone:
- Attacking a foe or affecting it with a skill.
- Healing or enchanting an ally which already holds aggro.
 Special circumstances
- Being the target of Enemies Must Die!.
- Being within spirit range of a Siege Wurm or Siege Turtle.
- Being seen by a Kournan Spotter (they can become aggressive even outside your radar range).
 Breaking aggro
Breaking aggro or losing aggro means reducing a player's threat level to make foes either switch their target or break off completely. Known ways of breaking aggro are:
- Inflicting multiple hits of damage from damage over time area of effect skills like Fire Storm, or Mark of Pain. This can cause foes to scatter out of the AoE. Foes that want to scatter but cannot move out of the AoE simply freeze and do not act at all.
- Moving faster in relation to a foe until the foe breaks off, by increasing the player's run speed or snaring that foe. For example, Escape, Dash, the Crippled condition, and many Water Magic hexes are well suited for this tactic.
- A closely related tactic is shadow stepping and, less common, teleporting out of a foes danger zone. Sometimes, shadow stepping/teleporting to just outside the danger zone is not enough to break aggro and foes might not break.
- Moving around at the same or even lower speed than a foe also decreases aggro, but takes longer and team members might have to assist by gaining aggro on that foe.
- Adding a high chance to block against physically attacking monsters.
- Knocking down a foe.
- Body blocking foes so they can't attack or use skills on their intended target.
 Aggro and allies
Some quests and missions that require keeping NPC allies alive (e.g. Thunderhead Keep, Gyala Hatchery, The Troubled Keeper) consist of allies that seem to draw a lot of aggro by foes. As players have little control over the actions of NPCs, these areas of the game can become challenging as healing and protection is spread out over more players, straining the resources of the healers.