Institution(s): 1. The Pennsylvania State University
I will review the history and current status of the [O III] λ5007 Planetary Nebula Luminosity Function (PNLF). It has been more than a quarter century since the PNLF was first used to measure the distance to a galaxy outside the Local Group. Since then, the PNLF has been applied to elliptical galaxies, spiral disks, galactic halos, and even to the intracluster populations of moderate-sized galaxy clusters. Yet a full theoretical explanation for the robustness of the method is still lacking. I will discuss the problems with the method, and focus on some of the latest clues as to why it works.