Introduction to thermal field theory

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.