Significant achievements have been made at the cross-roads of physics and planetary science. In the second half of the twentieth century, the discipline of planetary sciences has witnessed three major episodes which have revolutionized its approach and content: (i) the plate-tectonic theory, (ii) human landing and discoveries in planetary astronomy and (iii) the extraordinary technical advancement in high P-T studies, which have been abetted by a vast improvement in computational methods. Using these new computational methods, such as first principles including ab initio models, calculations have been made for the electronic structure, bonding, thermal EOS, elasticity, melting, thermal conductivity and diffusivity. <P> In this monograph, the boundaries of the definitions of a petrologist, geochemist, geophysicist or a mineralogist have been willfully eliminated to bring them all under the spectrum of "high-pressure geochemistry" when they deal with any material (quintessentially a chemical assemblage) - terrestrial or extraterrestrial - under the conditions of high-pressure and temperature. Thus, a petrologist using a spectrometer or any instrument for high-pressure studies of a rock or a mineral, or a geochemist using them for chemical synthesis and characterization, is better categorized as a "high-pressure geochemist" rather than any other kind of disciplinarian.<P> The contents of this monograph bring together, under one cover, apparently disparate disciplines like solid-earth geophysics and geochemistry as well as material science and condensed-matter physics to present a thorough overview of high pressure geochemistry. Indeed, such interdisciplinary activities led to the discovery of new phenomena such as high P-T behaviour in metal oxides (e.g. Mott transition), novel transitions such as amorphization, changes in order-disorder in crystals and the anomalous properties of oxide melts.