Xamarin.Forms in Visual Studio

I'm building an Android App in Visual Studio and I want to use the classes within the namespace Xamarin.Forms. The compiler doesn't recognize this namespace (see below), and it says I'm missing an assembly reference. Can you point me to the correct reference?

using Xamarin.Forms; // this statement generates and error: "type of namespace Forms does not exist in namespace Xamarin, are you missing an assembly reference?"

Thanks for your help,
Mike8