Silverlight 2 error: "The type or namespace Page could not be found"

This bit of code in app.xaml.cs causes the error:

        private void Application_Startup(object sender, StartupEventArgs e)
        {
            // Load the main control
            this.RootVisual = new Page();
        }

Which is strange because I think that is part of the template - so maybe I have set things up incorrectly.
I have

using System.Windows.Browser;

I thought it was because there was no reference to System.Web at first, but that can't be used in a Silverlight app I discovered.

ANy help appreciated.
Thanks