The short answer is that yes, with tools like Omnisharp and Xamarin Studio, you can write code targeting the CLR on OSX and Linux. Omnisharp basically interact

The short answer is that yes, with tools like Omnisharp and Xamarin Studio, you can write code targeting the CLR on OSX and Linux. Omnisharp basically interacts with Rosyln (the .NET compiler) to provide integration for various editors. The longer answer is that it really depends on what type of project you're looking to do. Certain technologies from Microsoft will never get ported. WPF and anything that relies on the System.Web assembly are the more well known exceptions. This is because both of those technologies rely specifically on Windows technology to work. WPF, for example relies on a library called PresentationCore which ultimately ends up using DirectX for rendering. System.Web, if I remember correctly, relies on specific IIS technologies to work. ASP.NET and .NET are moving to more platform independent states, which is what is on GitHub. The ASP.NET ream removed the requirement on System.Web and instead defined the OWIN standard which formalizes the communication between sites that are developed in .NET and servers that can run them. The 2 most widely used implementors of that are IIS 8 (with the HttpPlatform Handler), Suave (F#), and project Katana, although I know I'm missing one.