[Zope] Pre & post processing

I'm wondering what the "best" way to pre and/or post process all requests to either the entire Zope server or (better) a particular sub-tree of the content. I want to be able to verify that the request contains particular headers, that the session object is set-up with certain defaults, etc. etc. I'd like to be able to redirect the request if it doesn't meet certain requirements (at minimum, by raising an exception which ends up rendering standard_error_message). Suggestions? RTFM responses welcomed (as long as they specify what to read). Thanks, A.