This qualitative study draws on and extends two important concepts of the public understanding of science literature: scientists’ understandings of ‘the public’, and the currently popular notion of science-public dialogue. Using data from interviews with scientists and engineers, the ‘understanding of the public by scientists’ is examined. Constructions of publics are also an emphasis in a detailed case study of informal science-public dialogue at the Dana Centre, London. Here structures of power and authority are more traditional than the dialogue movement’s rhetoric would suggest, but there is also continual resistance to and contestation of these structures. To conclude, the threads of analysis are drawn together to provide a theoretical reasoning of, and model for, informal dialogue processes which are able to utilise the sociological complexity of scientists’ talk.