This volume tells the untold story of how observations of the cosmic microwave background radiation were interpreted in the decades following its serendipitous discovery, before the Hot Big Bang model became the accepted orthodoxy. The authors guide the reader through this history, including the many false trails and blind alleys that occurred along the way. Readers will discover how the Big Bang theory was shaped by alternative theories that exposed its weaknesses – including some that persist even today. By looking carefully at what it takes to reject an incorrect theory and the assumptions and processes at each stage, the authors examine the epistemological factors at play between an emerging scientific orthodoxy and since discarded alternatives. Their analysis of the cosmic microwave background provides a uniquely well-documented case study of theory building for a wide readership spanning cosmology, the history of physics and astronomy, and the philosophy of science more broadly.