S ince the middle of the 19th century, when thermodynamics was estab-lished as one of the most rigorous and profound areas of chemistry and physics, it is well known that the classical variables for describing the behavior of any system are composition, temperature and pressure. Nevertheless, when dealing with solid state synthesis -the so-called ceramic method-because most processes are carried out at high temperatures under ambient pressure, in most phase diagrams only changes in composition associated with temperature are considered and pressure-related data are scarce. Thus, this third dimension, pressure, remains to be fully explored for the synthesis of ceramic materials. Some unique aspects of pressure as a thermodynamic variable have already been discussed in former chapters. From a quantitative point of view, the pressure range is extremely wide: from the 10−32 bar existing in interstellar space to the 1032 bar at which some processes take place inside some stars and from the 10−6 bar attained in ultra-high vacuum chambers, common in condensed-matter laboratories, to the 106 bar that many materials can handle without problem at room temperature. We should mention that what we call ambient pressure (1 atmosphere ∼1 bar) is an exceptional situation in the universe, where most of the existing matter, because of gravity, supports very high pressures close to 9 GPa (9 × 104 bar) or higher. The Earth is a huge and dynamical high pressure and high temperature laboratory and very important physico-chemical changes take place deep in its interior. Finally, energy changes associated with pressure changes in a system are small and directly related to the system’s compressiblity. This explains why pressure is a relevant thermodynamic parameter in geology, astronomy, physics, chemistry, materials science, architecture, biology, and food science and technology.