Widely applied methods of electromagnetic power density measurement were presented and analyzed in this chapter. The considerations have a purely theoretical character, and they are concerned with the field relationships only. The source of the measurement accuracy limitation, by way of E- or H-field measurement (method error), was demonstrated, and the magnitude of the error for different combinations of sources and measuring probes was estimated. Then, a certain accuracy improvement, as a result of simultaneous E- and H-field measurement, and a calculation of an arithmetic or geometric mean was proposed.