I have .net application (GUI as well as PowerShell) built against 4.5. My OS is server 2012. When I attach my application to 2013 visual studio, the debugger is not working
Please note that while the accepted answer probably will fix the problem for now, it's best to be aware of the drawbacks of this solution. Making this change will make VS 2013 use the older style debugger for all you projects. It is a global setting. There are other ways to locally change this for a single project. Please read here for more info on this:
http://blogs.msdn.com/b/visualstudioalm/archive/2013/10/16/switching-to-managed-compatibility-mode-in-visual-studio-2013.aspx