This chapter talks about the various meanings of participation in relation to expert knowledge and about the various dimensions along which the public evaluate technical expertise. It suggests that scientific models for policy purposes are a particularly fruitful site for studying science and expertise in their public contexts. The work on expertise has concentrated on case-study analyses that show how, in particular circumstances, the objectivistic, universalizing claims of scientific experts unravel or appear inadequate. Public responses to the air-quality model are more diverse and complex than Funtowicz and Ravetz's work might lead us to suppose. Members of the public try to assess not only the epistemic and practical value of the model but also its opportunity costs and its likely political functions. The public assessment of the monitoring and modeling system appears to be still more wide-ranging or multidimensional than Funtowicz and Ravetz suggest.