Jeff,
When you say "adding a Page" we must then assume you are referring to an
ASP.NET web application. In that case you probably would be better served
posting your question to the ASP.NET newsgroup, not the C# language group.

If you go to the QUICKSTARTS at the asp.net site, you can find all kinds of
info and sample code on how to set up Authentication, Roles and Profies using
the built-in provider mechanism. There is an Active Directory provider too.
-- Peter
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To a certain extent it's up to the application to implement authorization and
authentication.

You can use code access security (CAS) to limit what a user can do; but
you'll either have to test or use exceptions and catch the
SecurityExceptions.  Plus, if your code is FullTrust some of those CAS
attributes simply won't be tested.

There's a few interfaces that help you implement application-specific
authorization and authentication.  See System.Security.IIdentity and
System.Security.IPrincipal.  If you want to base your authorization on a
Windows login (for which Windows would handle the authentication) then you
can have a look at WindowsIdentity and WindowsPrinicpal.  See also
System.Threading.Thread.CurrentPrincipal.