The book provides a critical analysis of the work carried out by various researchers in the field of studying non-stoichiometric compounds, summarizes the information accumulated in the periodic literature on their structure, chemical and physical properties, and considers various theoretical explanations for the existence of these interesting compounds. The first two chapters are devoted to structural analysis and statistical thermodynamics, without knowledge of which it is difficult to understand the material set out in the following chapters. Each chapter is written by a specialist in this area, which increases the scientific value of the monograph. The Russian translation was edited by Professor K.V. Astakhov, Doctor of Chemical Sciences, who provided the text with some notes. The scientific monograph is of interest to a wide range of scientists - inorganic chemists, physical chemists, crystal chemists, both theorists and those involved in the synthesis and analysis of complex compounds. It will be a valuable tool for graduate and senior students in the chemical and geological departments.