Apply conditional formatting to a cell when it is left BLANK

Hi Fellow Excellers,

I'm trying to apply conditional formatting (shading) to cells that are left blank.

(Purpose: I am designing a research template for a client to complete with data and want the spreadsheet to show them where they've "missed a bit"!)

Does anyone know how to do this?

(When I go to the conditional formatting box, it asks me to specify when "cell value is"..."between/not between/equal to/not equal to" etc. But there's no option to specify when the cell is blank.)

Thank you!

Rach