A summing up by one of the greatest of modern astronomers, this book assesses the state of our knowledge of the processes of development in stars and galaxies. The author's historical introduction shows how the modern picture of the universe sprang from the pioneer work of the Hertzsprung-Russell diagram, Shapley's study of globular clusters, and Hubble's attack on the galaxies. The chapters on the classification and observation of galaxies are rich in the experience of a great observer. They lead into a description of Dr. Baade's discovery of the two stellar populations, and of the consequent revision of the period-luminosity relation, which doubled the distance scale of the universe.