In less than a century, the accepted picture of the universe transformed from a stagnant place, composed entirely of our own Milky Way galaxy, to a realm inhabited by billions of individual galaxies, hurtling away from one another. We must thank, in part, Edwin P. Hubble, one of the greatest observational astronomers of the 20th century. In 1936, Hubble described his principal observations and conclusions in The Realm of the Nebulae, which quickly became a classic work. Two new introductory pieces, by Robert P. Kirshner and Sean M. Carroll, explain advances since Hubble’s time and his work’s foundational importance. "Meaningful, historically accurate, and thoroughly delightful reading."—Gail O. Clark, Astronomy