Planetary nebulae and the extragalactic distance scale

The [O III] gimel 5007 planetary nebula luminosity function (PNLF) is a key component of the extragalactic distance ladder, as it is the only standard candle capable of measuring distances to all the large galaxies of the Local Supercluster. We review the calibration of the method, and compare its results to those of the Surface Brightness Fluctuation method and Type la supernovae. We show that small systematic errors still exist in sonic of the methods of the distance ladder, and suggest that the canonical distance to the LMC, which sets the entire extragalactic distance scale, may be slightly too large. We also discuss why the PNLF has been so difficult to model, and argue that coalesced binary stars must play an important role in defining the bright end of the luminosity function.