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A variety of backscattering structures and grids is discussed to achieve frequency selection or/and polarization sensitivity. Theoretical concepts are reviewed numerically to compute co- and crosspolar reflection and transmission coefficients depending on parameters such as signal frequency, angle of incidence and grid orientation, grid geometry, and dielectric substrate. Special attention is given to technical applications in terms of dipole gridded antenna reflectors. Specific features such as focussing or defocussing effects can be achieved by using grids with an inhomogeneous dipole distribution or with electrically tunable dipoles. Methods of manufacturing and a tolerance analysis are outlined. Results of experimental investigations are presented and reviewed from free space anechoic chamber measurements and from radiation measurements using a wave guide simulation technique. An outline is given for future applications of active composite reflectors in microwave antenna technology.