Emergency services are taking part in a multi-agency exercise at York Racecourse today (Tuesday, 1 October).
Police are advising people not to be worried if they see lots of activity involving emergency services staff and vehicles.
The exercise is designed to test the response to a major incident and will involve the police, fire and ambulance services along with a number of volunteers who will play the part of casualties.
Most of the exercise will take place this afternoon between 1pm and 3.15pm.
A North Yorkshire Police spokeswoman said:
Emergency services work together at real incidents all the time but say it is still very important to test the response to rare, major incidents involving lots of casualties.