Namespace UI does not exist in System.Web error

Anyone know what could be causing this? Trying to create a custom web part. Following chapter 7 in MS ASP.NET 2.0 Step by Step book. Code on CD that came with book gets the same errors.
The type or namespace name 'UI' does not exist in the namespace 'System.Web' (are you missing an assembly reference?)
Get the error on all lines that include the UI:
using System;using System.Collections.Generic;using System.Text;using System.Web;using System.Web.UI;using System.Web.UI.WebControls;using System.Web.UI.WebControls.WebParts;
...