This book is devoted to the fundamentals of the theoretical analysis of phase equilibrium diagrams. Phase diagrams are known to play an important role in met- allurgy and materials science, chemical engineering, petroleum refining, etc. A study of phase diagrams can help in choosing the optimal composition of mixtures and alloys and in determining the appropriate conditions for their thermal treatment, as well as in determining the efficiency of such processes as distillation, rectifica- tion, zone refining, and controlled crystallization for the separation and purifi(;ation of materials. In spite of this, the extensive thermodynamic information which can be extracted from phase diagrams has scarcely been utilized until recently, due to the of the analysis of phase equilibria. comparatively poorly developed foundations We have attempted to present a general picture of the thermodynamic analysis of phase diagrams, and to demonstrate the broad possibilities of this approach in elucidating the nature of the interaction of the components and the structure of the phases. This book summarizes research carried out at the Moscow Institute of Electronic Engineering over the past decade. Extensive summaries of published data are also included. In the course of our work we have made extensive use of modern computing methods, which allowed solutions to be obtained to many problems.