I have a main form with a sub form. They are linked. Within the sub form I am popping up another form. I want to take the primary key field from the sub form and populate an ID field on the pop up form. Is there any way to do this?

Forms!SomeForm!MyField=Me.MyField

If you're trying to bring up your popup form to the same record as your subform and main form, look into the Access help topic, 'Synchronize records between two forms'.

HTH,
David R