Aggressiveness (or hostility) refers to the behaviour in RuneScape Classic in which some monsters will attack players automatically if they come within a certain range. This heeds players more in Classic than in RuneScape 2, due to the combat system in Classic preventing players from retreating for the first 3 rounds of combat. Aggressive monsters will also usually follow a retreating player for a certain distance.
However, with most aggressive monsters, if a player reaches a combat level that is one more than twice the monster's level, it will not be aggressive to that player any more. This rule does not apply inside the wilderness.
In RuneScape 2, there is a rule wherein monsters which are normally aggressive will stop being aggressive after a player has been in their vicinity for a set time. This rule is not present in RuneScape Classic. This makes AFK training more viable.
On this wiki, monsters are described using 5 levels of hostility:
|0||Pacifist||The mob cannot be attacked, and thus is usually referred to as a non-player character instead of a monster.||Sheep|
|1||Retreats||This is the level at which a non-player character becomes a monster: there is an "attack" right click option, players can attack it, but it will retreat when its hits is significantly reduced. A hostile player will then have to chase it to receive experience and dropped items.||Unicorns|
|2||Combative||The monster can be attacked by players, and will not retreat when its hits is reduced.||Jonny the beard|
|3||Aggressive||The monster can be attacked, will not retreat, and will attack nearby players with a combat level twice to its own or lower.||Blue dragons|
|4||Safe spot||The monster is only found in sealed-off areas, cannot attack, and can only be killed with Ranged or Magic.||Target practice zombies|
|5||Other||Everything else.||Alomone, Wyson The Gardener, Sir Mordred|