The present knowledge on grain boundary-related phenomena specific for severely plastically deformed materials is reviewed and critically analyzed in detail. Severe plastic deformation is shown to introduce specific metastable states of the grain boundaries which are characterized by enhanced diffusion rates, high-density of specific structure elements and large (still localized) elastic strains. An intrinsic heterogeneity of the deformation-induced modifications is revealed and examined on different scales. Relations between the existence of deformation-modified grain boundaries, specific microstructure features and resulting properties are highlighted.