Introduction to thermal field theory

This is intended to be a very basic introduction and short (and of course incomplete) overview of thermal field theory. In the first part, I introduce the thermal propagator at a very simple level and give the Feynman rules using the time-path contour method. In the second part, I give examples of these rules in scalar theory and discuss the origin of the thermal mass and other important effects as infrared divergences and phase transitions. In the third part, I outline the resummation program of Braaten and Pisarski.