List of illegal characters for element, attribute, and

I'm preparing some training materials for a group without much exposure to XML. I started to write about what form a name (for an element, attribute, or variable) can take. Specifically, what characters are not permitted. It was easy at first, but then I realized I should find an authoritative source. I googled, but came up empty. Either I asked for the wrong thing, or I couldn't recognize the right thing when I saw it. Can someone point me to a resource? Thanks.

-- 
Charles Knell
cknell@xxxxxxxxxx - email