In the years between the two world wars, the parties of the left had to confront new and frightening movements which were intent on their complete destruction. Fascism triumphed in Italy, Germany, Spain and elsewhere, coming to power after intense struggles with the labour movements of those countries. Yet in Britain, the left was able to face down the challenge of fascism by developing accurate analyses of the nature of the threat, and evolving tactics which were crucial to the failure of British fascism to grow and develop as it had elsewhere. How did socialists and communists in Britain explain what fascism was? What did they do to oppose it, and how successful were they? In examining the theories and actions of the Labour Party, the TUC, the Communist Party and other, smaller left-wing groups, this work seeks to explain their different approaches, while at the same time highlighting the common thread that ran through all their interpretations of fascism. While there are thousands of books on fascism itself, there are far fewer which examine the thoughts and actions of those who opposed it. The author argues that the British left has largely been overlooked in the few specific studies of anti-fascism that exist, with the focus being disproportionately applied to its European counterparts. He also takes issue with recent developments in the study of fascism, and argues that the views of the left, often derided by modern historians, are still relevant today.