  The [O III] 5007 planetary nebula luminosity function (PNLF) occupies an
important place on the extragalactic distance ladder: it is the only standard
candle that can be applied to all the large galaxies of the Local Supercluster.
We review the method's precision, and use it to show that the distance scale
defined by Cepheids and the Surface Brightness Fluctuation method is likely too
large by ~ 7%. We also discuss some of the physics underlying the phenomenon,
and present clues as why the technique is so resilient.
