My thesis is an account of the 'Women's Corps movement': the efforts to organise women's non-industrial employment, which led to the establishment of women's military services in Britain during the First World War. I survey the background to their introduction both before and during the First World War, and consider the development of pre-war women's organisations in the context of official thinking about defence. The outbreak of war prompted the formation of numerous women's voluntary organisations, and a number of women worked to extend women's role, but it was the continuing manpower crisis which, in 1916, persuaded defence ministers and others seriously to consider forming corps of women to substitute for men in the Army. The recommendations of both the Manpower Distribution Board and a military report advocating substitution of women in certain jobs, together with the desire of senior War Office staff to gain control over women's voluntary groups working for the Army, combined to secure the formation early in 1917 of the first of the three women's military corps, the Women's Army Auxiliary Corps. This was followed soon after by the Women's Royal Naval Service and the Women's Royal Air Force, and I review some of the problems which inevitably accompanied this innovation, such as the spreading of rumours of immorality in the corps, and the manner in which a lack of formal status created difficulties for the women in command. I examine the decision that the Corps should cease to exist after the First World War; the attempts during the 1930s to reorganise a women's corps to work for the armed forces; the formal bestowal of 'military status' upon members of the Auxiliary Territorial Service and Women's Auxiliary Air Force in 1941, and, finally, with the Army and Air Force (Women's Services) Act in 1948, the inclusion of women's services in Britain's peacetime defence organisation.