In the world of data acquisition if there is one thing which causes more anguish that anything else, it is grounding! Some sensor manufactures and most instrument providers include a provision for a third wire either as part of the signal output or the signal input path. The third wire has a variety of labels, shield, guard, common, ground, signal ground etc. Often, the signal wires are shielded and contain a “drain wire” adding to the connection complexity. This article attempts to provide a “simple” (if ground loops could ever be called simple) explanation of this phenomenon.