>I have a table that contains two classifications of items: products & services.
>When it is a 'service' I need to be sure that a Header record exists for it.
>To that end, I have a FOREIGN KEY on Item.HeaderID referencing Header.HeaderID.
>
>How can exclude the products, which have a NULL value in Item.HeaderID, from this FOREIGN KEY CONSTRAINT?
>
>Thanks in advance.

What is your database? SQL Server or VFP native tables?