It is particularly symptomatic that a volume concerning P-T-X phase equilibrium should appear in the Materials Science Series. Entering the 21st century, progress in modern electronics is increasingly becoming associated with devices based not only on silicon but also on chemical compounds. These include both semiconduc­ tors and, in the last 15 years, multinary oxides with high-To superconductor properties. The critical role of chemical processes in the technologies of these materials is quite evident, and in recent years has stimulated vigorous research activity in the physical chemistry of materials, resulting in a renaissance of this field. The leading role in these efforts belongs to thermodynamics, in particular, computer modeling of chemical processes, phase equilibrium, and controlled synthesis of inorganic materials with preliminary fixed stoichiometric composition. Especially important contributions have been made regarding non­ stoichiometry and our understanding of the crucial relationship between composition and properties of the materials since the development of the vapor pressure scanning approach to the phenomenon of non-stoichiometry. This method of the in situ investigation of the crystal composition directly at high temperatures 3 4 proved to be of an unparalleled precision of 10- _10 at. % and made it possible to obtain in an analytical form functional dependences of the crystal composition on temperature, pressure, and composition of the crystallizing matrix for crystals with sub-O. l at. % range of existence.