Planetary nebulae as probes of stellar populations

Planetary nebulae (PNe) have the potential to revolutionize our understanding of extragalactic stellar populations. Indeed, in many systems, bright PNe are the only individual objects identifiable from the ground, and, even more often, they are the only stars that are amenable to spectroscopy. We review the ways in which ensembles of PNe can be used to probe the metallicity, age, and history of a stellar population. In particular, we discuss three cases: the weak line spectroscopic regime, where one has knowledge of the line-strengths of faint forbidden lines such as [O III] λ4363, a bright-line regime, where only the strongest emission lines are visible, and the photometric regime, where the only information available is the [O III] λ5007 luminosity function. We show that each of these cases, when properly calibrated, can provide unique insights into the objects that make up a stellar population.