Pressure is an important thermodynamical variable. It provides the most efficient means of altering interatomic distances while leaving the thermal energy of a system invariant. It therefore provides an important mechanism for testing theoretical models that are based upon atomic separations and crystallographic configurations. Like its counterpart, temperature, pressure can be used to assist chemical reactions or to bring about crystallographic phase transformations. New allotropes, formed under conditions of extreme pressure and/or temperature, may have physical properties that are significantly different from those of the material formed under normal conditions. A classic example is that of carbon: the hardness, electrical and thermal conductivities, transparency, and cost of graphite, the normal phase of carbon, are significantly different from those of diamond, the phase formed at elevated pressures and temperatures. In the quest for higher static pressures, researchers have been reducing the size of the pressure chamber, and hence the sample, to microscopic dimensions; this, in turn, necessitates the use of brighter light sources to "see" the sample in a reasonable time period.