Advances in perturbative thermal field theory

The progress in the last decade in perturbative quantum field theory at high temperatures and densities, made possible by the use of effective field theories and hard thermal/dense loop resummations in ultrarelativistic gauge theories, is reviewed. The relevant methods are discussed in field theoretical models from simple scalar theories to non-Abelian gauge theories including gravity. In the simpler models, the aim is to give a pedagogical account of some of the relevant problems and their resolution, while in the more complicated but also more interesting models such as quantum chromodynamics, a summary of the results obtained so far is given together with references to a few of the most recent developments and open problems.