In the 25 years since the revelation of the so-called 'Ultra secret', the importance of codebreaking and signals intelligence in the diplomacy and military operations of the Second World War has become increasingly evident. Studies of wartime signals intelligence, however, have largely focused on Great Britain and the United States and their successes against, respectively, the German Enigma and Japanese Purple cipher machines. Drawing upon newly available sources in Australia, Britain, China, France and the United States, the articles in this volume demonstrate that the codebreaking war was a truly global conflict in which many countries were active and successful. They discuss the work of Australian, Chinese, Finnish, French and Japanese codebreakers, shed new light on the work of their American and British counterparts, and describe the struggle to apply technology to the problems of radio intercept and cryptanalysis. The contributions also reveal that, for the Axis as well as the Allies, success in the signals war often depended upon close collaboration among alliance partners.