The introduction describes the scope of the book. It starts by noting that it is a hundred years since Edwin Hubble published the observations showing there are stellar systems of similar size to the Milky Way, and well outside it: the external galaxies. It then mentions Hubble’s other key observation: the universe of galaxies is expanding at a rate proportional to the distances to the galaxies. Key observational signposts regarding galaxies and their relation to the universe as a whole, made as the twentieth century progressed, are then noted: the presence of excess gravitational mass in individual galaxies and in clusters, and the fact that from observations of the galaxies to ever increasing distances we find that the expansion of the universe appears to be accelerating. The structure and evolution of galaxies is probed using the properties of their individual stars for nearby galaxies and of their complete stellar populations for galaxies at increasing distances. The galaxies are used to test theories of the overall structure of the universe, over times stretching back over more than ten thousand million years, back to within a few hundred million years of the Big Bang which originated the expansion. As always in science, observation and theory are intermeshed. The book shows how far we have come since Edwin Hubble, and indicates the way forward to improve our knowledge and understanding with new telescopes and projects.