The progress of the last decade in perturbative quantum field theory at high
temperature and density 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 are given together with references to a few most recent
developments and open problems.Comment: 84 pages, 18 figues, review article submitted to Reports on Progress
in Physics; v2, v3: minor additions and corrections, more reference