If you previously installed a beta or Release Candidate (RC) of .NET 4.5, uninstall it via Control Panel > Programs and Features. Otherwise, installation will fail.
Install the .NET Framework developer pack or redistributable
Reference assemblies needed to build apps that target the specific framework version in Visual Studio.
Understanding what this pack enables helps contextualize its importance:
He found the link on a dusty Microsoft support page. With a click, the 72MB installer began its journey. He watched the progress bar with the intensity of a man watching a heart transplant.
Troubleshooting