Shielding efficiencies for a number of blends containing a variety of conductive fillers, including intrinsically conductive polymers, have been measured in the near-field with a dual-chamber box and in the far-field with a transmission line fixture. Although all samples tested satisfied the classical good conductor approximation, most of them exhibited a cross-over from being electrically thin (thickness < skin depth) to being electrically thick (thickness > skin depth) over the frequency range of interest, 1 MHz to 3 GHz. The theoretical relations for both near-field and far-field shielding which are prevalent in the literature do not accurately describe this region of crossover. Expressions are defined that describe the behavior accurately over the entire range of interest. Far-field shielding efficiencies as high as 70 dB at 1 GHz were measured for purely organic composites consisting of an intrinsically conductive polymer dispersed in a thermoplastic matrix.