Modern ideas and concepts of new class of materials - metal multi-component high-entropy alloys (HEA) are outlined in the monograph. Comparative analysis of technologies of HEAs manufacturing is carried out. The overview of published works on studying the HEAs structure, phase composition and properties in different structural states in a wide range of temperatures is performed. Features of HEA deformation behavior in mono- and polycrystalline states are distinguished. Problems of stability and perspectives of HEA application are analyzed. By the example of AlCoCrFeNi, CoCrFeMnNi systems the calculation of thermodynamical parameters and prediction of phase composition of quinary HEAs are considered. The results of experimental research in structural phase states and properties of the alloy carried out by the methods of modern physical materials science, are presented. The monograph is intended for specialists in the field of physics of condensed state, physical materials science, physical metallurgy and thermal treatment and it may be helpful for post-graduate students and graduate students of relevant specialties. The Research is supported by the Russian Scientific Foundation (project No. 20-19-00452).