I just inherited a project where one of the class files was "using System.Web.UI.Design;" My project did not compile of course, because I didn't have the assembly referenced. I had System.Web already referenced, so naturally I thought I'd add a reference to System.Web.UI and everything would be peachy.

Right?

Wrong.

There is not a System.Web.UI.dll or System.Web.UI.Design.dll to reference. After 5 wasted minutes I found the namespace in System.Design.dll.

So I decided to waste another 5 minutes writing this, and hopefully help someone else, in the future.