This new volume will give readers a complete history of the development of relativistic cosmology in the first half of the twentieth century. It traces the beginnings of the theory in 1917 with Einstein's first static model of the universe based on general relativity, and follows his conversion to the new cosmology after a series of controversial meetings with Dutch astronomer Willem De Sitter. The impact of these discussions on Eddington and Weyl, who later formulated the most fundamental principle of cosmology is examined, while the works of Friedmann and Lemaître, pioneers of the expanding universe theory, are covered in-depth. This valuable history will also provide insights on how and why the relativistic way of thinking contributes to some of the most enduring philosophical issues of our time.