A nationâ€™s defence R&D determines the international competitiveness of its defence industries and the technical superiority of its military forces. Whilst there is much secrecy, there are published data on defence R&D for many nations. Exceptions include China, Iran, Israel, North Korea, Pakistan and Russia. Defence R&D has led to rising unit costs of equipment and questions about the long-run affordability of some weapons. There are also external economic benefits but a lack of any measures of defence output. This article reviews what is known, what is not known and what is needed to be known for an informed debate and policy choices.