Planetary nebulae as tracers of galaxy stellar populations

We address the general problem of the luminosity-specific planetary nebula (PN) number, defined as alpha = N(PN)/L(gal), and its relationship with age and metallicity of the parent stellar population. Our analysis relies on population synthesis models for simple stellar populations and more elaborated galaxy models along the full star-formation range of the Hubble morphological sequence. This theoretical framework is compared with the updated census of the PN population in Local Group galaxies and external ellipticals in the Leo group, and the Virgo and Fornax clusters.