I thought I'd add one other point. The problem is not specific to the Ajax Control Toolkit's TabContainer. Indeed, I have a FormView control, and all the child controls in the ItemTemplate are no longer recognized by the compiler as being properies of the Page and result in compile errors. I can't believe that I have to change all of these propery references to findcontrol calls throughout my forms just because I moved from a Web Site to a Web Application Project. :-(