This contribution concentrates on studying diffraction characteristics in optical diffraction gratings. As modeling tools, both rigorous and approximate approaches have been analyzed and/or modified and successfully implemented. As for the diffraction characteristics, a general background is briefly presented, i.e. an idea of characterization of mechanisms and diffraction processes, classification of diffraction characteristics and regions with typical diffraction regimes. Different types of synchronisms as the parametrical dependences of the diffraction efficiency on the two of important parameters are discussed. Parameters chosen can either be of a grating or a mount type, thus defining different areas of applicability. A special role is devoted to the volume phase synchronism, i.e. a parametrical dependence of the efficiency on the relative structure period and relative structure/modulation depth. Then, extending the previous studies, description of the two types of grating classes, namely metallic and volume planar (holographic), based on the simulations, is given. Last part of the paper is devoted to new synchronism studies within resonant regions, namely to guided-mode resonance effect (reflection case). Apart from volume phase synchronism, parametrical dependences of the diffraction efficiency with respect to wavelength-duty cycle, wavelength-angle of incidence, wavelength-relief depth, wavelength-grating period, wavelength-polarization angle are given; the case of the conical mount is also discussed.