  We review the information that planetary nebulae and their immediate
progenitors, the post-AGB objects, can provide to probe the nucleosynthesis and
mixing in low and intermediate mass stars. We emphasize new approaches based on
high signal-to-noise spectroscopy of planetary nebulae and of their central
stars. We mention some of the problems still to overcome. We emphasize that, as
found by several authors, planetary nebulae in low metallicity environments
cannot be used to probe the oxygen abundance in the interstellar medium out of
which their progenitors were formed, because of abundance modification during
stellar evolution.
