This thesis explores major trends at work in international women's organisations and co-operation between the First World War and the Second World War. It analyses the changing compositions and aims of the International Council of Women, the International Alliance of Women, Women's International League for Peace and Freedom and the International Federation of University Women, and explores how far these shifts were demonstrated at their conferences and reflected in their journals. In particular it focuses on the experience of the women involved in these organisations, what "international sisterhood" meant to them (and there were differences in the ways that they interpreted this depending on factors relating to time and place), and, importantly, how notions of "sisterhood" were played out, and contested, within these organisations at this time. The first section establishes the historical framework and examines the evolution of these organisations in the interwar period. It places the development of these organisations within its broader context, in particular outlining the substructure of international Christian women's organisations formed earlier on which later developments were built. It then examines the unprecedented expansion of international women's organisations in the 1920s, and assesses the challenges experienced by them during the troubled 1930s. The second section is thematic, exploring themes of education, travel and regionalisation. It first highlights the significance of higher education for women's international co-operation in the interwar period and the role of the IFUW in particular. It then evaluates the importance of international travel for the expansion of international women's organisations, drawing attention to the changing function of travel during the first half of the twentieth century. Finally it addresses the increased regionalisation that had emerged by the end of the 1930s, comparing and contrasting the involvement of especially non-western women in, and their experience of, regional and international organisations.