Many of the world’s poorest countries can be described as 'fragile states' wherein governments cannot or will not provide an environment for households to reduce, mitigate, or cope with poverty and other risks to wellbeing. Many of these states are in conflict or just emerging from conflict. The UNU-WIDER project 'Fragility and Development' explored state fragility and its relationship to household vulnerability, noting that there is a lack of research on the economic dimensions of conflict, aid, and development in fragile states. This Research Brief provides a summary of the various contributions made by this project, including case studies on Iraq, Kosovo, Palestine, and Somalia. It also addresses a number of pertinent questions such as; when are states fragile? What are the costs that fragile states impose on their people and the international community? Should the sovereignty of fragile states be reconsidered? And how can aid flows to fragile states be made more effective?