  I use simple thermodynamic reasoning to argue that at temperatures of order a
trillion kelvin, QCD, the theory which describes strongly interacting particles
such as protons and neutrons under normal conditions, undergoes a phase
transition to a plasma of more elementary constituents called quarks and
gluons. I review what is known about the plasma phase both from theoretical
calculations and from experiments involving the collisions of large atomic
nuclei moving at relativistic speeds. Finally I consider the behaviour of
nuclear material under conditions of extreme density, and discuss possible
exotic phenomena such as quark matter and color superconductivity.
