The phase diagram of QCD

Simple thermodynamic reasoning is used to argue that at temperatures of the order of a trillion kelvin, QCD, the theory which describes strongly inter acting particles such as protons and neutrons under normal conditions, unde rgoes a phase transition to a plasma of more elementary constituents called quarks and gluons. A review is presented of what is known about the plasma phase both from theoretical calculations and from experiments involving th e collisions of large atomic nuclei moving at relativistic speeds. Finally the behaviour of nuclear material under conditions of extreme density is co nsidered, and possible exotic phenomena such as quark matter and colour sup erconductivity are discussed.