Mono is an open source implementation of the .NET CLR that allows .NET applications to run on non-Windows platforms. Mono for Android allowed building Android applications using C# and the .NET framework. It was released in 2011 but retired later that year. Developing Android applications with Mono required a Mac or Windows machine, Android and Java SDKs, and a Mono for Android license. Key aspects included activities, intents, navigation between screens, and databinding for lists.