This chapter describes the fundamentals of electromagnetism required for the design and application of electromagnetic wave absorbersElectromagnetic wave absorber. Electromagnetic spectrumElectromagnetic spectrum is introduced, with frequencyFrequency (wavelengthWavelength) ranges and applications for each frequencyFrequency band. This chapter also introduces the physical properties of materialsPermeabilityof materials in reaction to the external electric and magnetic fieldsMagnetic field and the associated material parameters (conductivityConductivity, permittivity, permeability) that establish the constitutive equations. SI and decibelDecibels units are introduced for representing the electromagnetic physical quantities and the gainGain or loss of powerPower (or voltage) in electronic devices. Sinusoidal harmonic wavesSinusoidal harmonic waves and their vector notation, phasorPhasor, are described together with their complex numberComplex numbers representation. Vector algebra and vector calculus are included for understanding and mathematical representations of electromagnetic field theory and related phenomena. Maxwell’s equations are introduced, including Faraday’s law related to electromagnetic inductionElectromagnetic induction, Ampere’s law to explain the generation of magnetic fieldsMagnetic field by conduction and displacement currentsDisplacement current, and Gauss’s law related to electric and magnetic fieldsMagnetic field. Maxwell’sMaxwell, J.C. equations in integral and differential forms have been described, expressing the above four electromagnetic phenomena.