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Grappled

Condition

A grappled creature's speed becomes 0, and it can't benefit from any bonus to its speed.
The condition ends if the grappler is incapacitated (see the condition).
The condition also ends if an effect removes the grappled creature from the reach of the grappler or grappling effect, such as when a creature is hurled away by the thunder-wave spell.

Monsters immune to it

Other conditions

Questions

How do you end the grappled condition?

It lasts as long as the effect that caused it says: a spell's duration, a repeated saving throw, or until something removes it.

Which monsters are immune to being grappled?

18 SRD monsters, including Air Elemental, Fire Elemental, Ghost, Invisible Stalker.