The equation of state was originally developed for ideal gases, and proved central to the development of early molecular and atomic physics. Increasingly sophisticated equations of state have been developed to take into account molecular interactions, quantization, relativistic effects, etc. Extreme conditions of matter are encountered both in nature and iq the laboratory, for example in the centres of stars, in relativistic collisions of heavy nuclei, in inertial confinement fusion (where temperatures of IO'K and pressures of several million atmospheres can be achieved). A sound knowledge of the equation of state is a prerequisite to an understanding of processes at very high temperatures and pressures.