Book excerpt: Recently, improved observational capabilities have allowed the study of fainter and fainter extra-galactic planetary nebulae in galaxies well beyond the Milky Way. This book result from a workshop held at ESO headquarters in Garching in 2004, the first devoted to Extra-galactic Planetary Nebulae. A wide range of topics is covered, from stellar and nebular astrophysics to galactic dynamics and galaxy clusters, making this a reference of broad astrophysical interest.