Science journalism, science museums, community outreach programs about science – all these forms of "public communication of science and technology" have a long history. But little is understood about the systematic differences between the goals and possibilities of different kinds of projects. This working papger identifies four key models that have been used to describe public communication activities: deficit model, contextual model, lay expertise model, and public participation model. It also identifies problems both within the models and with attempts to fit all activities into this particular set of models. It suggests both new areas for research and new possibilities for outreach.