Can't find ObservableCollection ... look in WindowsBase

I wanted to use ObservableCollection for databinding in WPF cause it handles all the nofications of list changes and generally is just cool. The problem i had was I had my objects in a different project..a dll and I couldn't access the ObservableCollection object. It worked if I was in a WPF project though. My problem was essentially that I didn't read the object definition in MSDN very well cause it clearly states it's in the WindowsBase directory! So if you're having this problem..you can't find ObservableCollectioin...just include WindowsBase.dll in the project references.