Question regarding Using directives

Getting started in ArcObjects 10.1 SDK...

Within Visual Studio 2010, I entered in the following for Using directives:
using ESRI.ArcGIS.ArcMapUI;
using ESRI.ArcGIS.Desktop;
using ESRI.ArcGIS.esriSystem;
using ESRI.ArcGIS.Framework;

I tried creating an IMaps variable by entering:
IMaps pMaps = pMxDocument.Maps;

But I am getting an error "The type or namespace name 'IMaps' could not be found (are you missing a using directive or an assembly reference?)". Based on Intellisense, I don't see what other directives I should use. This seems like it should have an obvious answer which I am missing.