where is the Sys.WebForms namespace?

I see it in the solution I am looking for, but if I try to use it I get:

CS0103: The name 'Sys' does not exist in the current context

I have VS 2008 SP1 installed, I am probably missing something that is pretty obvious?

From stackoverflow

• Sys namespace is in JavaScript. It's a client side namespace provided by ASP.NET AJAX. You can't use it from server side code.