The most basic material parameters for the design of EM wave absorbers are dielectric constantDielectric constant andLossdielectric dielectric lossDielectric loss, permeability andLossmagnetic magnetic lossMagnetic loss. These material parameters are expressed in complex numbersComplex numbers and are referred to as complex permittivityComplex permittivity and complex permeabilityComplex permeability. This chapter is about the dielectric polarizationDielectric polarization and magnetizationMagnetization of materialsPermeabilityof materials at the atomic and molecular levels, and the associated high-frequencyFrequency behavior is explained, including material loss. Dielectric polarizationDielectric polarization mechanisms (electronic polarizationElectronic polarization, ionic polarizationIonic polarization, orientation polarizationOrientation polarization, space charge polarizationSpace charge polarization) are described, with the addition of domain polarizationDomain polarization in ferroelectricsFerroelectrics and defect-dipole polarizationPolarization in ionic solids. Debye relaxationDebye relaxation andResonancedielectric dielectric resonanceDielectric resonance are described, which are the two main mechanisms for the electromagnetic absorption in dielectricsDielectrics. MagnetizationMagnetization is theDomainmagnetic processDomainelectric of domain wallDomain wallmagnetic movement andDipolemagnetic magnetic dipoleMagnetic dipole rotation. TheLossmagnetic magnetic lossMagnetic loss mechanisms generated in this process are divided into domain wall resonanceDomain wall resonanceelectric, gyromagnetic resonanceGyromagnetic resonance, and magnetic relaxationMagnetic relaxation. In each case, the theoretical derivation of complex permeabilityComplex permeability is described, and the dominant loss mechanisms depending on the materials and frequencyFrequency are discussed.