Considerable progress has been made in our understanding of the physicochemical evolution of natural rocks through systematic analysis of the compositional properties and phase relations of their mineral assemblages. This book brings together concepts of classical thermodynamics, solution models, and atomic ordering and interactions that constitute a major basis of such analysis, with appropiate examples of application to subsolidus petrological problems. This book is written for an audience with a senior undergraduate level background in chemistry. Derivations of fundamental thermodynamic relations which are in need of reemphasis and clarification are presented.