session variable only accessable through page behind class instance?

Are session variables available to any class instance within a project namespace or only for use for the form codebehind class?

I have tried to set a session variable on a code behind class and then retrieve its value in a class instance used by the codebehind class and I get errors. Maybe I'm just not meant to access that way and only codebehind classes (classes that inherit System.Web.UI.Page) can use them?As far as I know session variables are only available in aspx pages.

That does not prevent you from passing value of your session variables to any of your business tier objects.
It's more a question of context.
Within the context of the ASP.NET process, any class can get to the session using HttpContext.Current.Session.
Thanks very much for the help!