Since this is a Tizard Memorial Lecture, I think it is appropriate that I should tell you something about Operational Research in the Royal Air Force as it is carried on today. The RAF has a great tradition of actively using operational research to promote the efficiency of the Service and it was Sir Henry Tizard who initiated this tradition and sustained it until it was well established. Indeed, he was responsible for the much wider application of science to air warfare, but I am not attempting to cover the wider field in this lecture.