We have an WPF Application build on .net framework 3.5. Some testers find if they uninstall .net framework 3.5, but install .net framework 4.0, our APP fails to launch itself. Dose this mean that .net framework 4.0 does not include all 3.5 libs, and users have to install .net 3.5 even though they have 4.0? I see here are some migration issues listed by Microsoft http://msdn.microsoft.com/en-us/library/ee941656.aspx#windows_presentation_foundation_wpf Are they all breaking changes so that the backward compatibility is ruined? Thanks