name does not exist in the current context

I have read the msdn posts on this problem, but they all seem to be referring to VS2005.  I'm running VS2008 on Vista.

 I have a web form page with various input fields and a submit button, but in the code behind none of the controls are visible. The intelliSense does not recognize the field ID tags, and the build results in the errors 'The name xxxx does not exist in the current context.'
I do not understand why the code behind does not work with:
btnUpdate.Text = "Insert";

aspx code:

<%@ Page Language ...