Yeah, you can't just say "Field Value Is" and then put NULL for the value. You have to use an Expression and say

IsNull([FieldName])

One of the members recently asked me this in the TechHelp bin. I'll be making a video on it soon.

I get an invalid syntax message when I try =IsNull([FieldName]).

What EXACTLY are you putting in the box?

IsNull([FieldName]) is example you need to replace [FieldName] with the actual Field you wish to check.

Yes I did put the actual fieldname.. None of the following work though they pass the syntax test:

Value = Null
Value = ""
Value = 0
Value = IsEmpty([CheckDate])
Value = IsNull([CheckDate])

FieldName is CheckDate a date field

These are rules for conditional formatting.

Apologies
Out of curiosity, this doesnt use Conditional Formatting but would be interested to know if this works:
Blog: Access Tip: Check a Form for Blank Controls
Tip: Check for Empty/Null Control

Did you include the word "Value"? Just start with the equal sign.

Correction to my last post. Just did a test - Leave off the equal sign.

Here you go: Conditional Formatting Null