In the monograph, the first of this type in the world, the authors discuss systematically the current state of investigations into nanocrystalline materials. The experimental results on the effect of the nanocrystalline state on the microstructure and the mechanical, thermophysical, optical, and magnetic properties of metals, alloys and solid-phase compounds are generalised. Special attention is given to the main methods of production of isolated nanoparticles, ultrafine powders and dense nanocrystalline materials. The dimensional effects in isolated nanoparticles and high-density nanocrystalline materials are discussed in detail, and the important role of the interface in the formation of the structure and properties of dense nanocrystalline materials is shown. The modelling considerations, explaining special features of the structure and anomalous properties of substances in the nanocrystalline condition, are analysed.