The study of policy transfer initially focused on transfers and transmissions among developed countries or from developed countries to the developing world. Today the circulation of policy and knowledge has become more dense and complex. The articles in the special issue concentrate on the growing velocity of policy innovations spreading from the developing world to other parts of the developing as well as into developed countries and towards international organisations. The context of international development cooperation has been particularly fertile in the cross-pollination of ideas, models and policy experiments, and the articles in this Special Issue draw deeply on this insight. Using a ‘development lens’ enables the authors to view processes of knowledge diffusion and policy transfer not from the centre, in the ministries of national governments, but from policy perimeters, in cities and local government, among those outside political power in opposition groups and movements, and bottom-up from policy implementers.