In the years between 1900 and 1931 astronomers witnessed three startling changes in their view of the Universe. First, the accepted value of the size of the star system, which increased by a factor of ten; secondly, evidence forced the acceptance of the fact that there are other star systems beyond our own Galaxy; and lastly, that observation of these external galaxies disclosed the expansion of the Universe. This book, originally published in 1982, describes and explains in detail these shifts in opinion, considering them in the light of theories and ideas on the nature of the Universe, were current at the beginning of the twentieth century. Archive material is used to provide major interpretations of several of the processes and events associated with these shifts such as the ‘Great Debate’ between Harlow Shapley and H. D. Curtis in 1920 on ‘The scale of the Universe’. This book with be of interest to professional and amateur astronomers as well as historians of science.