In the context of materials, there is probably no other broad subject that impinges more on the various aspects of materials science and technology than the description of phase stability in terms of temperature and composition, known as a Phase Diagram. Phase diagrams are the "road maps" that guide and direct us to our numerous goals in fabrications, developments, heat treatment, properties and basic science. In this presentation, I shall discuss briefly the three laws of thermodynamics, including considerations of how phase diagrams are determined and assessed, and how they can be judged in terms of thermodynamic parameters. I shall then turn attention to metastability and concepts that arise when, because of kinetics, ideal equilibrium conditions can be suppressed or altered by heat treatment, such as cooling, rapid quenching, annealing etc. The use of phase diagrams and opportunity which they present for development of unusual properties, or structures, or features will be considered next, and will be followed by expectation of phase behavior, and phase diagram features, at temperatures between 0C and 0K, where the third law becomes increasingly important and kinetics are reduced. Finally, the process of assessing phase diagrams through the examination of published work on experimental determinations, on modeling calculations, and on measured thermodynamic quantities will be considered. Here, I will stress the usefulness of international cooperation in this whole area.