Building an app for multiple platforms has typically required multiple development tools. Before the advent of Xamarin.iOS, development for the iPhone could only be done on a Mac using Xamarin Studio (previously MonoDevelop, which was a port of the open source SharpDevelop). While there’s nothing wrong with Xamarin Studio, developers typically want to stay within the same IDE as much as possible. Visual Studio 2013 used with Xamarin.iOS for Visual Studio lets you develop for Windows Azure, Windows Phone and the iPhone without having to leave the IDE you know and love.