This book is about the Dark Energy Survey, a cosmological experiment designed to investigate the physical nature of dark energy by measuring its effect on the expansion history of the universe and on the growth of large-scale structure. The survey saw first light in 2012, after a decade of planning, and completed observations in 2019. The collaboration designed and built a 570-megapixel camera and installed it on the four-metre Blanco telescope at the Cerro Tololo Inter-American Observatory in the Chilean Andes. The survey data yielded a three-dimensional map of over 300 million galaxies and a catalogue of thousands of supernovae. Analysis of the early data has confirmed remarkably accurately the model of cold dark matter and a cosmological constant. The survey has also offered new insights into galaxies, supernovae, stellar evolution, solar system objects and the nature of gravitational wave events. A project of this scale required the long-term commitment of hundreds of scientists