Don't be alarmed if you see the army and gardai swarm the streets of Dublin this morning.
Defence chiefs have issued a notice to say they'll be carrying out a major training exercise.
We don't know where it is or at what time, because that's being kept under wraps.
The public can expect to see military and armoured vehicles, as well as helicopters and armed gardai.
The simulated terror attack will allow the elite Army Ranger Wing to practise in case there's ever a real emergency.
In June, officers carried out a dramatic training exercise in the north inner city.