The monograph for the first time gives a systematic presentation of the current state of research on nanocrystalline materials. Experimental results on the effect of the nanocrystalline state on the microstructure and mechanical, thermophysical, optical, magnetic properties of metals, alloys and solid-phase compounds are summarized. Basic methods of producing isolated nanoparticles, ultrafine powders and compact nanocrystalline materials are considered.