Electromagnetic metamaterials (MTMs) are broadly defined as artificial effectively homogeneous electromagnetic structures with exotic properties not readily available in nature. They consist of an arrangement of subwavelength scattering particles emulating the atoms or molecules of real materials with enhanced properties. The scattering particles are typically arranged in a periodic lattice with the unit cell size p < λ g $p\ll\lambda_g$, where λ g is the guided wavelength inside the MTM. Under such operating conditions, the structure behaves as 166a real material so that the electromagnetic waves sense the average, or effective, macroscopic and well-defined constitutive parameters, which depend on the nature of the unit cell. Their 2D counterparts are known as metasurfaces, which are thin layers of subwavelength resonant scatters that interact strongly with electromagnetic waves for achieving unique wavefront processing functionalities.