The application of effective medium theory to a description of a temperature and composition dependence of conductivity of composite polymeric electrolytes is presented. Conductivities of several composite systems containing conducting (NASICON) or nonconducting (Theta-Al2O3, polyacrylamide) additives are analyzed in terms of effective medium theory approaches. All of the systems studied are based on polyether matrices. The influence of grain size distribution, concentration and type of additives on conductivity of composite systems is discussed. The model presented assumes that an increase in the conductivity in comparison to polyether based electrolytes is due to the formation of highly conductive layers at the polyether matrix-filler interface. Variation of the conductivity of this layer with grain sizes, and the concentration of a filler is assumed and discussed. The theoretical assumptions are confirmed by experimental data obtained by several techniques, such as impedance spectroscopy, DSC, NMR and energy dispersive X-ray diffractometry.