.NET Framework
We can refer the 1.1 framework compiled dlls in Web applications developed in 2.0 without any extra effort. However we can not refer the 2.0 compiled dlls in 1.1 framework application the reason is well illustrated in the thread at ASP.NET Migration forum which says : using COM Interop it won't work because even if it uses COM for "interoperating" it still needs the .NET Framework to run. Since you cannot load 2 versions of the .NET Framework in the same process it will end up running against .NET...