  The propagation of uncertainties in reaction cross sections and rates of
neutron-, proton-, and $\alpha$-induced reactions into the final isotopic
abundances obtained in nucleosynthesis models is an important issue in studies
of nucleosynthesis and Galactic Chemical Evolution. We developed a Monte Carlo
method to allow large-scale postprocessing studies of the impact of nuclear
uncertainties on nucleosynthesis. Temperature-dependent rate uncertainties
combining realistic experimental and theoretical uncertainties are used. The
importance of contributions of cross sections of reactions on excited states of
the nuclear targets, which have weights different from from the thermal
Boltzmann population factors, is explained. From detailed statistical analyses
of the Monte Carlo data uncertainties in the final abundances are derived as
probability density distributions. Furthermore, based on rate and abundance
correlations an automated procedure identifies the most important reactions in
complex flow patterns from superposition of many zones or tracers. The method
already has been applied to a number of nucleosynthesis processes.
