  Within the next few years experiments at RHIC and the LHC will seek to create
in the laboratory a quark-gluon plasma, the phase of matter in which the
Universe was initially created. It is believed that the plasma will survive
long enough to reach thermal equilibrium. I give an introduction to the
formalism of thermal field theory, the combination of statistical mechanics and
quantum field theory needed to describe the plasma in thermal equilibrium, in a
way that tries to keep close to the physics it describes.
