The scattering of waves from an object is a diffraction process that transforms the incident wave into the waves propagating from the object in all possible directions. The backscatter, or backcattering, is the scattering of waves back toward the source of the incident wave. This process substantially depends on material properties of the scattering object, its shape, size, and spatial orientation relative to the incident wave, as well as on the frequency and polarization of the incident wave. One distinguishes three frequency regions with different physical properties of scattered waves. They are quasi-static, resonance, and quasi-optical regions.