This thesis examines the origins and development of signals intelligence in the Royal Australian Navy, during the period 1914 to 1945, within the context of an Australian contribution to Imperial defence. In doing so it demonstrates how the development of this capability was shaped by national, Imperial and international forces and events. The thesis thus fills a gap in the historiography of imperial defence and of early twentieth century signals intelligence. It also constitutes a case study of the development of a niche military capability by a small to medium power in the context of great power alliances and major historical events. The thesis is based principally upon the investigation of documents in the Australian, US and UK national archives, some of which have been newly declassified for this purpose. During the First World War the RAN undertook a minor cryptographic effort focused on intercepting and breaking coded messages from the German Pacific Squadron. After the War, and at the request of the RN, the RAN began to develop a signals intelligence capability aimed at the Imperial Japanese Navy. This capability was seen as part of the RAN contribution to Imperial defence. The commitment, made without Australian political approval, would see the RAN conduct two covert intelligence collection operations against the Japanese Mandated Territories. After the Japanese attack on Pearl Harbor and subsequent defeat of the Western Powers in Asia, the RAN signals intelligence organisation became, as a consequence of agreements between Britain and the USN, part of the USN organisation in the Pacific. At no stage however, was the RAN involved in the discussions which accompanied these arrangements nor did it have any subsequent say in the strategic direction of this capability. As a consequence, when the Pacific War was drawing to a close the future of the RAN's cryptographic organisation came in to question. By the time the Japanese surrendered this issue had still not been resolved. Beyond the history of the origins and development of signals intelligence in the RAN, and of its involvement in the signals intelligence war against Japan, the thesis highlights the importance of committed individuals in small military organisations and how they can greatly influence the success or otherwise of these organisations. The ability of personnel from different nations to work together in signals intelligence is reflective of the functioning of the alliance as a whole. The development of such a niche capability by a small to medium power can have an effect on that nation's standing, in the context of alliance relationships, as it did in this case. As the RAN found however, such capabilities do not provide for automatic access to strategic decision making within an alliance.