This chapter gives detailed description of the microwave–material interaction. The classification of materials based on microwave–material interaction is presented. The different kinds of polarizations such as dipolar, interfacial, and electronic are elaborately incorporated in this chapter. The dielectric polarization losses, etc., responsible for volumetric heating are talked about in detail. The dependence of the relative permittivity and loss tangent of a material on frequency is covered. The concept skin depth and its role in MW heating of the metallic materials in powder form are discussed in detail.