Severe plastic deformation (SPD) has often been applied for refining the microstructure of polycrystalline materials down to the submicron grain size range, yielding bulk materials that are mostly free from porosity or contaminations of the internal interfaces. At the same time, materials that were processed by severe deformation routes showed remarkable property modifications that exceed the modifications due to the mere presence of the total area of internal interfaces. Thus, the presence of “deformation-modified” grain boundaries has been advocated and recently also shown experimentally. Their presence, their structure and particularly their strain state is of importance for the stability, the mechanical performance and, apparently, also for selected functional properties of these materials. The present contribution will summarize results of microstructure -, mechanical - and diffusion analyses and discuss possible scenarios concerning the correlation between SPD processing and the resulting modifications of microstructures, grain boundary diffusion characteristics and macroscopic materials properties.