Forgotten how to do optional criteria in a report

[Sandy H]

Hi
I've done this before but can't remember how I did it.

I have a report that is opened after entering criteria into a Criteria form.
I want some of the criteria to be optional.

The report's query specifies the criteria with expressions like:
forms!frmrptCriteria!EstateID

How do I write the criteria so that if the criteria is null, the report
still works.

I hope this makes sense.

Thanks in advance
Sandy