Capacity building is a pervasive idea that has received little critical treatment from anthropology. In this introduction, we outline the growing use of the idea of ‘capacity building’ within and beyond development settings, and highlight mechanisms by which it gains footholds in both policy and practice. This special issue centres and questions its histories, assumptions, intentions and enactments in order to bring ethnographic attention to the promises it entails. By bringing together cases from different sectors and continents, the collection pursues capacity building’s self-evident character, opening up what capacities themselves are thought to be. By not taking capacity building’s promises for granted, the articles collected here have two aims: to interrogate the means of capacity building’s ubiquity, and to develop critical purchase on its persuasive power.