yes, its a part of the host. It is called by the unmanaged code of the exe-file and than decides depending on the required .net version (e.g. 1.0.3075) which host (e.g. mscorwks.dll or mscorsrv.dll) to use. It is a wrapper that the .net-exes can be easily started. You can examine a .net-exe compiled on windows and you will see that mscoree.dll is referenced