  Planetary nebulae (PN) represent the evolutionary fate of the asymptotic
giant branch (AGB) stellar envelopes, thus are ideally suited to study the
chemical impact of AGB stars. Stellar evolution predict elemental enrichment
through the AGB evolution, and convective dredge-up episodes allow the products
of stellar evolution to reach the stellar outer layers. Planetary nebulae are
probes of these processes, and are also probes of the environment at the time
of formation of their progenitors, through the elements not affected by AGB
evolution. Ultimately PN may be used to test AGB stars as actors and probes.
Planetary nebulae are easily identified and detected in the galaxy, the
Magellanic Clouds, and beyond, thus they are probes of AGB evolution an stellar
populations in different environments as well.
