Today I have deployed a .net 4.o website which was upgraded from 2.0 sometimes ago, and found that the ReportViewer that was created back in VS2005 time no longer worked; the error returned was to do with that the Microsoft.ReportViewer.ProcessingObjectModel.dll could not be found; the ReportViewer worked normally on my local machine and I used the Web Deployment project to wrap things up and xcopy needed files to an internal web server. I guess the deployment package did not pickup what was needed for ReportViewer; as it turned out, after reading some useful posts here http://forums.asp.net/t/1723107.aspx/1 , I needed to download the ReportViewer 2010 redistributed package ReportViewer.exe from here http://www.microsoft.com/download/en/details.aspx?displaylang=en&id=6442; after I ran the ReportViewer.exe on the deployed web server, the error was fixed and gone.