As a result of the Paris and Accra declarations, over 100 countries have committed themselves to a new model of partnership, in which donors and partner countries hold one another mutually accountable for development results and aid effectiveness. But mutual accountability remains little explored in conceptual and practical terms. This Background Paper explores this issue through the lens of research commissioned by the German development agency (GTZ) to clarify the concepts and highlight emerging good practices in 19 countries, with a special focus on Rwanda, Mozambique and Vietnam. It suggests that a mutual accountability system does not exist at present, and may not be desirable given the variation of context. It iargues for stronger and more balanced mechanisms at national and international level; between development partners as well as those partners and their domestic constituents.