This chapter describes the design methods for different types of EM wave absorbers based on equivalent circuit modelsEquivalent circuit model and simulation techniquesSimulation technique. All EM wave absorbers of various geometries, from plane to pyramid or from monolayer to multilayer with or without periodic conductive elements, can be represented as equivalent electrical circuits, allowing for the design of optimal structures with easier mathematical processing. As one of the simulation techniquesSimulation technique, HFSS is introduced, including the entire simulation process. Starting from the conventional resonant-type absorbers (1/4 wavelengthWavelength absorber, Salisbury screenSalisbury screen, etc.), we review the latest metamaterialMetamaterials and frequencyFrequency selection surface (FSS) absorbers with more advanced design techniques. Equivalent circuit method is introduced to determine the circuit parameters of FSS (inductanceInductance L, capacitanceCapacitance C) as a function of the shape and dimensions of the periodic conductorConductors elements. Recent research on how to design ultrawide band absorbers by multilayering FSSs with different dimensions or patterns is also included. Another design technique for wide-bandwidth absorbers describes a method for controlling the shape of a lossy material (grid-type, reverse grid-type, and pyramid-shaped absorbers). Finally, the design method for oblique incidence absorbersOblique incidence absorbers is included with the results of simulation and experiment for single-layer and multilayer structures.