I f one asks for a definition of high pressure (HP), the answer will certainlydepend on the scientific environment in which the question arises: physics, chemistry, materials science, geophysics, biology, or food technology, just to name a few areas in which the pressure variable plays an important role. Such uncertainty in the answer also appears if we consider other variables like temperature or the electric and magnetic fields, so the term “extreme conditions” is often used to refer to extreme values of all these variables within the

context of the phenomena under investigation. Among our scientific community, there is a certain agreement in that high pressures refer to the study of those phenomena occurring above 1000 bar (kbar). Other terms like “ultrahigh pressure” were coined to refer to phenomena involving pressures above 1000000 bar (Mbar). Of course, the above limits are somewhat arbitrary and the fundamental laws studied in other chapters apply uniformly regardless of the range of pressures considered. This means that we share methodology with many other scientific areas involving only tens or few hundreds of atmospheres. In other words, the same fundamental thermodynamics and kinetics models apply to all phenomena in which the pressure variable is considered.