This special section explores the various ways in which states of certainty and doubt are generated and sustained, focusing on what we call the ‘infrastructures’ that undergird, enable or develop alongside them. The articles in this collection build on the growing literature on these topics, notably the very extensive recent work on doubt, uncertainty and opacity, and they extend it further by directing attention not to the consequences of these states or people’s responses to them, but instead to the various semiotic, material and social forms that make possible the assertion or recognition of certainty or doubt. We use the idea of ‘infrastructure’ as a heuristic device to explore these processes.