Note: This article was originally published in 2011. Some steps, commands, or software versions may have changed. Check the current .Net documentation for the latest information.
The answer can be found if you look at the error message when ASP.Net can’t find the assembly. Below is a list of places it would look for the assembly for Telerik.Web.UI:
LOG: Attempting download of new URL file:///C:/Windows/Microsoft.NET/Framework64/v2.0.50727/Temporary ASP.NET Files/root/7b4cc07f/99d5b9e0/Telerik.Web.UI.DLL.
LOG: Attempting download of new URL file:///C:/Windows/Microsoft.NET/Framework64/v2.0.50727/Temporary ASP.NET Files/root/7b4cc07f/99d5b9e0/Telerik.Web.UI/Telerik.Web.UI.DLL.
LOG: Attempting download of new URL file:///C:/inetpub/wwwroot/wss/VirtualDirectories/80/bin/Telerik.Web.UI.DLL.
LOG: Attempting download of new URL file:///C:/inetpub/wwwroot/wss/VirtualDirectories/80/bin/Telerik.Web.UI/Telerik.Web.UI.DLL.
LOG: Attempting download of new URL file:///C:/inetpub/wwwroot/wss/VirtualDirectories/80/_app_bin/Telerik.Web.UI.DLL.
LOG: Attempting download of new URL file:///C:/inetpub/wwwroot/wss/VirtualDirectories/80/_app_bin/Telerik.Web.UI/Telerik.Web.UI.DLL.
LOG: Attempting download of new URL file:///C:/Windows/Microsoft.NET/Framework64/v2.0.50727/Temporary ASP.NET Files/root/7b4cc07f/99d5b9e0/Telerik.Web.UI.EXE.
LOG: Attempting download of new URL file:///C:/Windows/Microsoft.NET/Framework64/v2.0.50727/Temporary ASP.NET Files/root/7b4cc07f/99d5b9e0/Telerik.Web.UI/Telerik.Web.UI.EXE.
LOG: Attempting download of new URL file:///C:/inetpub/wwwroot/wss/VirtualDirectories/80/bin/Telerik.Web.UI.EXE.
LOG: Attempting download of new URL file:///C:/inetpub/wwwroot/wss/VirtualDirectories/80/bin/Telerik.Web.UI/Telerik.Web.UI.EXE.
LOG: Attempting download of new URL file:///C:/inetpub/wwwroot/wss/VirtualDirectories/80/_app_bin/Telerik.Web.UI.EXE.
LOG: Attempting download of new URL file:///C:/inetpub/wwwroot/wss/VirtualDirectories/80/_app_bin/Telerik.Web.UI/Telerik.Web.UI.EXE.