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Chemical bonding between the constituent phases of composite materials is necessary to ensure that the mechanical reinforcement of the dispersed phase is transferred to the bulk properties of the composite. At a chemical structural level of a polymer composite, the interfacial region is comprised of molecules of the polymer matrix bonded to the surface of the filler particle. This interfacial-bonding region is termed the "interphase" and results from the confining effect that the rigid filler particles have on the mobility of the polymer molecules in the matrix. In composite systems containing a large concentration of filler particles or a high filler surface area, the properties of the interphase region may have a pronounced effect on the bulk properties of the composite. In this paper, a microstructural model called the interphase power law model (IPL Model) is described in which the physical, chemical and electrical characteristics of the interphase region may be determined. The IPL Model is based on a simple extension of a general power law model in which a composite system containing filler, interphase and matrix regions may be treated as a unique three-component composite system comprising two primary components (matrix and filler) and an interphase region that is inextricably dependent upon the characteristics of the filler and matrix components. The IPL Model provides a means to determine the interphase properties and volume fraction of the overall composite system. The cause and effects of the interphase region on a variety of complex composite systems is investigated. Effects of the composite filler types and filler surface areas as well as the dielectric characteristics of the interphase region are explored.