  We present a unified description of the high temperature phase of QCD, the
so-called quark-gluon plasma, in a regime where the effective gauge coupling
$g$ is sufficiently small to allow for weak coupling calculations. The main
focuss is the construction of the effective theory for the collective
excitations which develop at a typical scale $gT$, which is well separated from
the typical energy of single particle excitations which is the temperature $T$.
We show that the plasma particles provide a source for long wavelength
oscillations of average fields which carry the quantum numbers of the plasma
constituents, the quarks and the gluons. To leading order in $g$, the plasma
particles obey simple gauge-covariant kinetic equations, whose derivation from
the general Dyson-Schwinger equations is outlined. As a by-product, the ``hard
thermal loops'' emerge naturally in a physically transparent framework. We show
that the collective excitations can be described in terms of classical fields,
and develop for these a Hamiltonian formalism. The effect of collisions among
the hard particles is also studied, in particular in relation with the
effective theory for ultrasoft excitations, with momenta $\sim g^2T$.
