It has been said that all work is cooperative, and so any work supported by a computer is computer-supported cooperative work (CSCW). To some, such a position would seem to give a book on evaluation for CSCW a very wide brief: we would like to widen it still further. Our primary concern is how one can evaluate the wide range of electronic facilities now being provided to help people communicate and cooperate. These facilities range from email and on-line electronic conferencing to video links used as an adjunct to shared tools. Their purpose is to mediate conversations between people about work, and their use has been dubbed “mediated” communication in order to differentiate it from the more common face-to-face communication.