Low surface brightness galaxies

The properties of galaxies that are lower in surface brightness than the dark night sky are reviewed. There are substantial selection effects against the discovery of galaxies that are unevolved or diffuse; these systems are missing from most wide field catalogs. Low surface brightness galaxies make up a significant amount of the luminosity density of the local universe. They contribute substantial but poorly determined amounts to the census of baryons and dark matter. Low surface brightness galaxies are also relevant to the interpretation of quasar absorption lines and to the understanding of rapidly evolving galaxy populations in the more distant universe. Theories of galaxy formation and evolution must accomodate the properties of these diffuse stellar systems.