If I have a ListViewModel and the items are bound to a simple model like:

public class TestViewModel : ViewModelBase
{
public string Description
{
   get { return _description; }
   set
   {
      if (_description != value)
      {
         _description = value;
        OnPropertyChanged("Description");
      }
   }
}
} 
The list's item binding DisplayMemberPath is bound to Description.
The first time the list is loaded, everything is bound and displayed correctly. However, if I programatically change Description in the model, the text in the list does not update.
How do I get that to work?