  These lectures are an introduction to recent developments in the study of the
high temperature phase of QCD by using weak coupling techniques. After a brief
introduction to some techniques of quantum field theory at finite temperature,
I proceed to an analysis of the various important scales and degrees of freedom
of the quark-gluon plasma and focus on the effective theory for the collective
modes which develop at the particular momentum scale $gT$, where $g$ is the
gauge coupling and $T$ the temperature. A powerful technique to construct the
effective theory is based on kinetic equations which govern the dynamics of the
hard degrees of freedom. Some of the collective phenomena that are described by
this effective theory are briefly mentioned. Then I turn to the calculation of
the entropy of the quark-gluon plasma and show how the information coded in the
effective theory can be exploited in (approximately) self-consistent
calculations.
