Ultrahigh Pressure Metamorphism (UHPM) is a fast growing discipline that was established 25 years ago after discoveries of high pressure minerals, coesite and diamonds. The current explosion of research on UHMP terranes reflects their significance for understanding large scale mantle dynamics, major elements of plate tectonics such as continental collisions, deep subduction and exhumation, mountains building, geochemical recycling 'from surface to the core', and a deep storage of light elements participating in green-house effects in the atmosphere. This book provides insights into the formation of diamond and coesite at very high pressures and explores new ideas regarding the tectonic setting of this style of metamorphism.