This contribution concentrates on modeling of diffraction processes in optical diffraction gratings (ODG). First, the characterization of mechanism and diffraction processes is briefly presented, together with the regions with typical diffraction regimes. Different types of volume phase synchronism are then described, and their periodicity and characteristic properties are explained and physically interpreted. Different situations are analyzed and compared concerning ODG of different types (surface-relief, planar holographic, metallic), different refractive index/relief modulation profiles (binary, sinusoidal, triangular), various modulation strengths, and incident wave polarization influence. As rigorous modeling tools, both rigorous coupled wave analysis, and coordinate transformation methods are used, implemented and modified. Some new and interesting results concerning resonant and threshold effects in diffraction process are also presented.