Reduce, Reuse, Recycle: Planetary Nebulae as Green Galactic Citizens
  We review gas-phase abundances in PNe and describe their dual utility as
archives of original progenitor metallicity via the alpha elements, as well as
sources of processed material from nucleosynthesis during the star's evolution,
i.e., C, N, and s-process elements. We describe the analysis of PN spectra to
derive abundances and discuss the discrepancies that arise from different
choices at each step. Abundance results for the Milky Way and Magellanic Clouds
from various groups of investigators are presented; the observational results
are compared with theoretical predictions of AGB stellar yields. Finally, we
suggest areas where more work is needed to improve our abilities to determine
abundances in PNe.
