Galaxies are not uniformly distributed in space, but instead show a tendency to gather together in galaxy groups and clusters of galaxies. This effect can be clearly recognized in the projection of bright galaxies on the sky (see Figs. 6.1 and 6.2). The Milky Way itself is a member of a group, called the Local Group (Sect. 6.1), which implies that we are living in a locally overdense region of the Universe.