When faced with anything out of the ordinary, faulty or suspicious, the work of determining and categorizing the trouble, and scoping for what to do about it (if anything) often go hand in hand--this is diagnostic work. In all its expert and non-expert forms diagnostic work is often both intellectual and embodied, collaborative and distributed, and ever more deeply entangled with technologies. Yet, it is often poorly supported by them. In this special issue we show that diagnostic work is an important and pervasive aspect of people's activities at work, at home, and on the move. The papers published in this Special Issue come from a range of domains including, ambulance dispatch, a friendly fire incident and anomaly response for the NASA space shuttle; software, network and photocopier troubleshooting; and users attempting to use a new travel management system. These papers illustrate the variety of work that may be thought of as diagnostic. We hope that bringing a focus on diagnostic work to these diverse practices and situations opens up a rich vein of inquiry for CSCW scholars, designers, and users.