Electromagnetic (EM) wave absorbers are based on the fundamental principles in wave dynamics of EM waves and their interaction with materials. This chapter provides the circuit theoryCircuit theory, including transmission linesTransmission lines, to gainGain basic knowledge for modeling and interpreting these physical phenomena with electrical circuits. The electrical behavior of DC and AC circuits of passive devicesPassive devices (resistorResistor R, inductorInductor L, capacitorCapacitor C) is described through analysis of magnitude and phase between voltage and current. Subsequently, series and parallel circuitsParallel circuitsRLC of R, L, C are described by circuit parameters of impedanceImpedance and admittanceAdmittance. Transmission linesTransmission lines are useful for expressing the propagation of electromagnetic wavesElectromagnetic wave as equivalent electrical circuits. Based on the distributed element circuitDistributed element circuit theory, the transmission line equationTransmission line equations is derived in terms of the characteristic impedanceCharacteristic impedance and the propagation constantPropagation constant. The input impedanceInput impedance is defined in the load-connected transmission lineTransmission lines, which provides a circuit theoryCircuit theory that connects to the design of the EM wave absorber.