I have a form with a combo box that looks up a table field Job_Reference. This Job_Reference will be the foreign key in another table. I want to ensure that the user selects an existing job_reference record from the combo box (which then populates the field for the foreign key i another table) before he/she can do anything else on the form. In other words they can't move to another field until the job_reference number has been selected. How do I do this please?