Although human life is confined to a narrow range of pressure of a few atmospheres and scores of degrees of temperature, violent events and processes that occur at multiple orders of magnitude of these two variables are taking place continuously within our planet and the vast universe. From a molecular perspective, chemical events occur as bonds are broken and others are formed, with the nuclei circumventing energetic barriers. The effect of temperature is to accelerate the motion in crossing these barriers. The effect of pressure, however, is diabolic: it changes the structure of the barrier height, so while it is decreased in some cases, energetic barriers are increased in others. The combination of both high-temperature and pressure on a system, such as in hot and dense fluids, alters the chemical transformation in a manner that is markedly different from that which we encounter in gas-phase chemistry. Chemical processes that occur in the pressure regime of 0.5 - 200 GPa and temperature range of 500 - 5000 K include such varied phenomena as comet collisions, synthesis of super-hard materials, detonation and combustion of energetic materials, and organic conversions in the interior of planets.