There are two types of mocking frameworks for .NET. Most, like NMock, Moq, Rhino Mocks are written in managed code and cannot mock static, sealed or private methods. On the other hand Typemock Isolator, Telerick JustMock and Microsoft Moles are using the .NET Profiler API (using that requires some unmanaged code). The last 3 can even mock stuff from mscorlib, like DateTime.Now.