Since J.W. Gibbs in 1878 succeeded comprehensively in establishing the basic principles for an understanding of equilibria in heterogeneous systems, numer- ous books concerning constitution diagrams have been written, some of them providing a formal treatment of phase equilibria down to the small detail. The purpose of the present book is to provide an introduction to the practical ap- plications of phase diagrams. In the first instance it is intended for students of chemistry, metallurgy, mineralogy and materials science, but also for engineers and students of science and engineering disciplines concerned with materials. To facilitate the start of an involvement with heterogeneous equilibria, reactions and dynamic equilibria will be treated first, since these are familiar to chemists and metallurgists. Of course, a description of phase equilibria is not possible without a mini- mum of formalism. The formalistic description, however, will be made lighter by clear explanations of experimental methods used to determine the constitu- tion of a system, by application examples, as well as by discussing realistic cas- es from chemistry, metallurgy, materials science and mineralogy. By this, the ne- cessity of the knowledge of phase diagrams can be shown. On the other hand a practical exercise is possible.