 Harry Nussbaumer, Professor Emeritus at the Swiss Federal Institute of Technology, and Lydia Bieri, an assistant professor in the mathematics department at Harvard, have written an engaging history of our unfolding understanding of one of the most important discoveries of 20th-century astronomy—the expanding universe. Beginning with an overview of some seeds of modern cosmology planted in late medieval texts, they use primary documents to focus on the work of major contributors to the idea.