Some can. You can use Mono on Linux and Mac to run your .NET apps -
but, it isn't guarenteed to work. For instance, if you are using COM
interop or P/Invoke then your application will need to be modified. If
you are using some of the newer API's (such as WPF) then you may have
issues. The mono team has a tool that can scan your project and detect
potential porting issues - so it might be worth checking that out.