This month's installment completes our primer series on magnetic fields by defining conditions where remedial EM site shielding is in fact technically feasible and by illustrating examples of both passive and active shielding methods (or attaining lower site fields. For those readers who may have missed one or more previous articles, we have thus far presented some basic physics describing magnetic source fields [Microscopy Today, November, 1995), examined magnetic survey equipment and methods relevant to EM interference thresholds (Ml, January, 1996), and, in Part IV (“Survey Data Analysis”, MT, May, 1996), suggested techniques for interpretation of typical EM site survey data.