This book is intended for scientists, researchers, and graduate students interested in solutions in general, and solutions of metals in particular. Readers are assumed to have a good background in thermodynamics, presented in such books as those cited at the end of Chapter 1, "Thermo- dynamic Background." The contents of the book are limited to the solutions of metals + metals, and metals + metalloids, but the results are also appli- cable to numerous other types of solutions encountered by metallurgists, materials scientists, geologists, ceramists, and chemists. Attempts have been made to cover each topic in depth with numerical examples whenever necessary. Chapter 2 presents phase equilibria and phase diagrams as related to the thermodynamics of solutions. The emphasis is on the binary diagrams since the ternary diagrams can be understood in terms of the binary diagrams coupled with the phase rule, and the Gibbs energies of mixing. The cal- culation of thermodynamic properties from the phase diagrams is not emphasized because such a procedure generally yields mediocre results. Nevertheless, the reader can readily obtain thermodynamic data from phase diagrams by reversing the detailed process of calculation of phase diagrams from thermodynamic data. Empirical rules on phase stability are given in this chapter for a brief and clear understanding of the physical and atomistic factors underlying the alloy phase formation.