  The pre-equilibrium evolution of a quark-gluon plasma produced in a heavy-ion
collision is studied in the framework of kinetic theory. We discuss the
approach to local thermal equilibrium, and the onset of hydrodynamics, in terms
of a particular set of moments of the distribution function. These moments
quantify the momentum anisotropies to a finer degree than the commonly used
ratio of longitudinal to transverse pressures. They are found to be in direct
correspondence with viscous corrections of hydrodynamics, and provide therefore
an alternative measure of these corrections in terms of the distortion of the
momentum distribution. As an application, we study the evolution of these
moments by solving the Boltzmann equation for a boost invariant expanding
system, first analytically in the relaxation time approximation, and then
numerically for a quark-gluon plasma within the small angle approximation to
the collision kernel.
