A detailed model of diffraction of Gaussian beams on plane uniform volume Bragg gratings based on a Kogelnik"s theory of coupled waves is presented. The model describes transmitting and reflecting gratings and takes into account spectral width and angular divergence of diffracted beams. Exact formulas for angular and spectral selectivity are derived. Conditions for Bragg diffraction based on comparison between beam quality (divergence and spectral width) and volume grating parameters (angular and spectral selectivity) are formulated. The model results are compared with experimental data for high-efficient Bragg gratings in photo-thermo-refractive (PTR) glass.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.