The [O III] 5007A planetary nebula luminosity function (PNLF) occupies an important place on the extragalactic distance ladder. Since it is the only method that is applicable to all the large galaxies of the Local Supercluster, it is uniquely useful for cross-checking results and linking the Population I and Population II distance scales. We review the basics of the method, speculate on its underlying physics, and demonstrate its precision via internal and external comparisons. We then use the PNLF to search for systematic offsets in the Cepheid, Surface Brightness Fluctuation, and SN Ia distance scales. We conclude by suggesting other uses of PN luminosity functions, such as studying the mass distribution of PN central stars, and exploring the chemical evolution of galaxies.