  A nonequilibrium Green's functions approach to the collective response of
correlated Coulomb systems at finite temperature is presented. It is shown that
solving Kadanoff-Baym type equations of motion for the two-time correlation
functions with the external perturbing field included allows to compute the
plasmon spectrum with collision effects in a systematic and consistent way. The
scheme has ``built-in'' sum rule preservation and is simpler to implement
numerically than the equivalent equilibrium approach based on the
Bethe-Salpeter equation.
