Aggressiveness (or hostility) refers to whether monsters in RuneScape will attack nearby players without being provoked. Aggressive monsters may attack players without being attacked first; some are aggressive based on players' Combat levels and others are aggressive regardless of level such as various bosses and monsters in the Wilderness. Aggressive monsters can be particularly dangerous in a multi-combat area with multiple monsters. Non-aggressive, or passive, monsters will not attack players unless they are attacked first.
When aggressiveness is based on Combat level, an aggressive monster will attack a player whose combat level is less than or equal to twice the monster's level. If the player's combat level is more than twice the monster's level, it won't attack (i.e. double the monster's combat level plus one).
For example, a level 51 player enters an area with a level 28 Hobgoblin. Since the player's level is less than twice the monster's level (28 × 2 = 56, which is more than 51), the hobgoblin will attack the player. Conversely, if a level 105 player enters an area with a level 28 hobgoblin, the monster will not attack because the player's level is more than twice the monster's. In this case, the hobgoblin will ignore the player (unless, of course, the player decides to attack the monster). Thus, the player would need to be level 57 or higher to avoid being attacked by the hobgoblin.
Different monsters seem to have different levels of aggression; some, such as Jungle horrors, will seek you out from many spaces away, whereas others, such as Mountain trolls, are aggressive on a smaller range.
- After a player spends a certain amount of time (about 10–20 minutes) in the vicinity of aggressive monsters, the monsters will become tolerant of the player and become passive. However, if the player leaves the area and then returns, the monsters there are aggressive again. Note that logging out is not classed as leaving the area, so if a player logs out and logs back in again the monsters will still be tolerant.
- Some monsters that become tolerant may become aggressive again, after a period of time, even if the player constantly stays in the vicinity. For example, Flesh Crawlers and Zombies in the Stronghold of Security and Kurasks in the Fremennik Slayer Dungeon are this way. Other monsters do not spontaneously become aggressive again no matter how long the player remains in the vicinity.
- Most monsters in the Wilderness are aggressive regardless of level. Nearly all monsters on Ape Atoll exhibit this quality as well to players not holding a Greegree, but they will also cease being aggressive once Monkey Madness 2 quest has been completed. In addition, guards will attack players whom they catch thieving from stalls anywhere on RuneScape regardless of the player's combat level.
- Most Slayer creatures are passive, despite the player's combat level. This does not apply to all however, as Kurasks, Gargoyles, Dark beasts, and Aberrant spectres are aggressive.
- Desert bandits south of Bedabin Camp will always be aggressive to players wearing god items.
- Desert bandits (to players not wearing Saradomin or Zamorak items) and Al Kharid Warriors are both passive until a player attacks one. All of the other warriors or bandits in the vicinity will then attack that player because both areas are multi-combat.
- Creatures that never become tolerant provide excellent AFK training opportunities, such as Dark Beasts (90 Slayer required), Desert Bandits and all monsters within the Nightmare Zone minigame.