  In order to describe properties of an equilibrated quark-gluon plasma, QCD at
finite temperature (and density) has to be considered. Besides lattice
calculations, which can be applied only to static quantities at zero density,
perturbative QCD has been used. At finite temperature, however, serious
problems such as infrared divergent and gauge dependent results have been
encountered. These difficulties can be (partially) avoided if one starts from
effective Green functions constructed by resumming a certain class of diagrams
(hard thermal loops). Within the last few years this improved perturbation
theory (Braaten-Pisarski method) turned out to be a powerful tool for computing
interesting quantities of the quark-gluon plasma phase. In the present talk a
basic introduction to the Braaten-Pisarski method is provided and its
applications and limits are reviewed. In particular, damping rates, the energy
loss of energetic partons, thermalization times, the viscosity of the
quark-gluon plasma, and the production of photons and dileptons are discussed.
