This work is devoted to the generation of nanocomposites and stable nanocrystallites by severe plastic deformation (SPD) which is quite a new research area in materials science. The main research topics in the last years were the fundamental understanding of the fragmentation process of materials deformed by SPD at very high strains and the limitations of the refinement of single phase materials. The first chapter of this work is assigned to give a short introduction on the most commonly used SPD methods and in particular to the ones used to perform the experiments during this thesis. Furthermore, a short review about the deformation of single phase materials is given. The main parameters influencing the limitations in the refinement of single phase materials are outlined and a process is proposed which might be the reason for the limit of the refinement. While a lot of studies have been published about the above mentioned issue in recent years, the structural evolution of multiphase materials during SPD has not been extensively studied and documented. First experiments on metal-metal composites (Cu-Cr, Cu-W) were already performed which result in nanocomposites with a grain size around 10 nm. SPD of the individual materials used in the before mentioned composites leads to a much larger grain size. Therefore, SPD of composites might offer the possibility to obtain materials with a nanocrystalline grain size and offer a possibility to overcome the limitations in the refinement. A short review about all kinds of different composites produced by SPD is also given in the first chapter. During this thesis, it was possible to develop a new procedure to generate stable nanocrystalline materials by SPD using a similar approach as mentioned above. It is presented in the second chapter of this thesis in more detail. Instead of bulk composite materials, metal powders were consolidated and severely plastically deformed. Due to natural oxide layers on the powder surfaces, nanometer sized oxide dispersions were incorporated in the latter bulk compacts which allows the fabrication of nanocrystalline metal matrix composites. Moreover, not only oxide dispersions were used to stabilize the metallic matrices. By the use of carbon nanoparticles, stable nanocrystalline copper and nickel composites could be obtained as well. Furthermore, powder mixtures of different combinations of metal powders were severe plastically deformed to form nanocomposites of the individual constituents. Special attention is given to possible dissolution processes and formation of supersaturated solid solutions which were formed during processing although the elements chosen are normally immiscible. After annealing, nanocrystalline nanocomposites with enhanced hardness and thermal stability were obtained.