The paper traces the development of O.R. and systems analysis in the United Kingdom since the phrase 'operational research' was coined just before World War II. Despite confusion as to titles and definitions, the subject has grown to the stage where it is used as an aid to decision making in most sectors of industry and government, and where some 200 students receive post-graduate qualifications each year. Although the subject has a strong interaction with established disciplines, it can now claim to be a distinct area of scientific endeavour, bearing a similar relationship to those disciplines as does engineering to mathematics and the physical sciences. O.R. and systems analysis are usually considered as complementary to each other, but it is more correct to think of them as 'duals'. Thus O.R. is primarily concerned with the improvement of decisions which cannot, however, be studied effectively without a consideration of the systems within which they are embedded; and vice versa. There are a number of distinctive features of the O.R./systems approach in practice. One is that it seldom attempts to supersede the decision maker by proposing the answer to a given problem. More commonly a study is directed towards the improvement of the decision making process, e.g. by redesigning that process, by supplying better information or through the provision of improved analytical tools. A second distinctive aspect follows from a realization that the experience and understanding which lead to a decision on a complex issue is developed by a number of people over a period of time. The only way to aid the decision is to play a part in that development. O.R. and systems analysis can do this both through basic research and the establishment of relevant models, particularly if they can be used 'interactively' by those concerned. Successful research in this area requires anticipation of needs as well as the development of special skills, including the understanding of social systems behaviour. To conclude, the paper explores the need for research to improve the methodology of O.R./systems analysis and extend its range of application.