Append Dictionary onto an existing one

Hi,

I have an existing dictionary variable:

      Dictionary<string, string> itemList = new Dictionary<string, string> ();

And I would like to add the below one to it so the values are added to the end, its returned from a method.

       Dictionary<string, string> returnValue = new Dictionary<string, string> ();

ie somthing like:  itemList +=returnValue (I know this is wrong)