This book shows how environmentalists have shaped the world's largest multilateral development lender, investment financier and political risk insurer to take up sustainable development. The book challenges an emerging consensus over international organisational change to argue that international organisations (IOs) are influenced by their social structure and may change their practices to reflect previously antithetical norms such as sustainable development. This important text locates sources of organisational change with environmentalists, thus demonstrating the ways in which non-state actors can effect change within large intergovernmental organisations through socialisation. It combines a theoretically sophisticated account of international organisation change with detailed empirical evidence of change in one issue area across three institutions. The book will be of interest to academics, postgraduate and upper undergraduate students in international relations, international political economy, environmental politics, development and globalisation studies and geography as well as policy makers, international bureaucrats and development practitioners.