How to trigger an alert if annual leave dates have already been selected for another employee - MS Access?

Well there many ways to implement this..but we need some more info on how you have setup your monitor form.
I guess the simplest solution would be with the use of Conditional formatting...e.g painting Red all the employees with annual leave

Do you have weekly-based or daily-based annual leaves?

John - My monitoring form is attached to each individual employee, when I enter the start and end date of their annual leave, I would like a trigger to pop up to show if there are any other employees off at any period between those chosen start and end dates.

Als315 - annual leave can be entered for any dates.

can we have a screenshot because i think i am having some other kind of implementation

by the way if you have a kind of an individual form per employee then on the After Update of the dates text boxes you can have a query run to indicate the employees who currently are in a leave

Here you go John.
Annual-Leave-Form.JPG

I think the After Update should cover your needs...just place a check method that ensures that both Start and End Date are correctly filled and then run a recordset iteration that will inform you the employees with conflicting leaves...if you want to check before entering the actual leave then a small form with similar functionality (just start,end date) should be sufficient

That's great, thank you very much.

You're welcome.