Diffraction of plane waves by infinite gratings is a classical research topic in the scattering theory. Using the Floquet theorem, one can reduce the infinite grating problem to the one-period problem. A characteristic feature of infinite-grating scattering is the drastic transformation of the scattering pattern and reflectance intensity if, in the process of changing the frequency or the angle of incidence, one of the Floquet harmonics is "passing over horizon." This phenomenon was first explained by Rayleigh [1] who studied theoretically the "anomalies" discovered experimentally by Wood [2]. In the simplest case of the normal incidence, these Rayleigh-Wood anomalies are observed if the period of the grating is multiple to the wavelength. © 2010 IEEE.