About the book, the project Equilibrium Between Phases of Matter - Phenomenology and Thermodynamics is a textbook, in which the phenomenology, the thermodynamic theory, and the practical use of phase diagrams are presented in three levels that diverge in nature - in particular as regards the role of thermodynamics. The book has been written from a chemical and geological teaching background. Each of the three levels of the book is representative of a particular course in a curriculum. Level 0: an introduction to phase diagrams The philosophy behind the ground level is that most of the characteristics of equilibrium between phases can be understood without the use of thermodynamics, realizing that, in a common-sense manner, the experimental observations on equilibria and spontaneous changes, and elementary notions about interactions, indicate the way to go. In spite of all this, the central figure in level zero, right from the beginning, is the chemical potential - a concept firmly rooted in thermodynamics. Equilibrium conditions in terms of chemical potentials, and the variables necessary to define a system in equilibrium are, are the basic elements of the system formulation.