The term nebulae means on the one hand clouds of gas and dust which are distributed among the stars. Within the Milky Way system, there are perhaps a few thousand of these galactic nebulae, their distribution exhibiting a decided preference for low galactic latitudes. The remaining objects, numbering hundreds of millions within the reach of the 200-inch telescope, are stellar systems distributed through extragalactic space. These systems, which, because of the absorption of light through the dust near the galactic plane, are mostly found in higher galactic latitudes, have been variously called island universes (Herschel), extragalactic nebulae or external galaxies. We shall refer to them simply as galaxies.