I want to add a reference to IronPython.dll to my project.
I found this dll in C:\Program Files (x86)\IronPython 2.7\ and different dll's for different Net versions in C:\Program Files (x86)\IronPython 2.7\Platforms\ . What is the difference between these dll's and which one shall I use? 
The same story with Microsoft.Scripting.dll & Co.
The assemblies in the root (C:\Program Files (x86)\IronPython 2.7\) are for ipy.exe to use. Apps that embed IronPython should use the ones in the appropriate Platform directory (or NuGet, which will pick the right ones automatically).
For now, the assemblies in the root are identical to the ones in Platforms\net40, but that is not in any way guaranteed and will almost certainly change in the future.
IronPython.dll - which one to use?