In this article a theoretical background for understanding dielectric phenomena in heterogeneous polymeric composites is presented, and the application of these principles to practical problems is illustrated by experimental examples. First a short overview of the existing theoretical mixing formulae is given, the differences in these approaches are discussed, and the areas of application are indicated. Conditions for the appearance of an interfacial relaxation process are delineated; limitations of the theories are given. Experimental examples include immiscible polymer blends, mineral-filled polyolefins, thermoset composites, and conductive composites. In the first example, AC dielectric spectroscopy is used to monitor the phase separation process and to evaluate interactions between the separated phases. In the second example, the effect of filler permittivity, volume fraction, and surface treatment is discussed with respect to the AC and DC dielectric properties. It is shown that in mineral-filled polyolefins, water adsorbed at the matrix-filler interface plays an important role. In the third example, the effect of filler purity (and conductivity) on the dielectric properties is demonstrated. Nonelectrical-grade glass powder causes a strong interfacial polarization and electrode polarization in an unsaturated polyester resin, whereas electrical-grade glass fiber reinforcement leads to minor property changes. In the final example, the effects of processing conditions on the AC conductivity and permittivity of conductive composites is discussed. Extensive shear and orientation cause a shift of the percolation threshold, thus reducing the conductivity. Thus it is shown that dielectric methods can be used to investigate problems in processing technology.