Development assistance to fragile states and conflict-affected areas can be a core component of peacebuilding, providing support for the restoration of government functions, delivery of basic services, the rule of law and economic revitalization. Despite a wealth of research, however, significant gaps remain in our knowledge about what has worked, why and the transferability and scalability of findings. The project presented in this special issue offers new research on ‘successful’ projects and programmes in diverse domains and contexts in an effort to address these gaps. This article introduces the special issue and eight case studies included in this volume. Three broad factors are highlighted as significant in understanding why some interventions work better than others: the area of intervention and the related degree of engagement with state institutions; local contextual factors such as capacity and the existence of local supporters; and programme design and management. The article concludes with discussion of how these case studies and similar research can speak to broader debates in the literature on how peace is built, and in particular on the interaction between external actors and endogenous processes.