An Assist is a play performed out-of-turn in response to a particular Trigger. Each Assist has its own Trigger text that defines the circumstances that permit the Assist to be played; these are usually worded in response to ally activities. In addition to the requirements in the Trigger text, an entity can only play an Assist if it is not their turn and they have not already played an Assist this turn. Some Assists and effects may explicitly override these rules.
Assists may also trigger outside of any entity's turn, such as during the round's Draw Phase or Discard Phase. Only one Assist may be played per phase in this manner, including only one during the Turn Phase outside of any entity's turn.
There is no limit to the number of Assists played per round. An entity may play an Assist during every other entity's turn if it has the ability to do so and the Trigger is met.
Stat Tracks may prevent an entity from playing Assists or permit additional plays within a turn or phase.
The DM's Guide has details on resolving the timing of Assist plays.
See also: Interrupt