This is an open access title available under the terms of a CC BY-NC-SA 3.0 IGO licence. It is free to read at Oxford Scholarship Online and offered as a free PDF download from OUP and selected open access locationsInternational development cooperation plays a key role in promoting social, economic, and political progress and wellbeing in developing countries. Yet, it has long been clear that the effectiveness of aid is most challenging in countries with weak institutions. It is estimated that by 2030 around a fifth of the world's poorest people will live in fragile and conflict-affected countries. Despite increased attention, much more needs to be done to maximize the effectiveness of aid in these highly complex situations. This book offers fresh, in-depth understandings of the effectiveness of development cooperation in fragile contexts, and where challenges regarding the effectiveness of development cooperation are most pronounced. The empirical chapters combine quantitative macro analysis of cross-country data with detailed country case studies to take stock of past research on the topic, investigate the impact of foreign aid in fragile state contexts, and explore whether and how aid delivered in accordance with the principles of effective development cooperation works in these settings. Cross-national studies provide a broader picture of the topic, while country case studies allow us to identify specific ways in which development cooperation operates. More reflective chapters discuss the feasibility of the effectiveness agenda and provide important insights on how the effectiveness principles could be improved and better put into practice. The collection offers important original contributions to a large body of policy-relevant scholarship on the aid-development nexus, as well as to rapidly evolving research and policy debates on building strong and inclusive state institutions.