Although public engagement is now part of the business of doing science, there is considerable divergence about what the term means and what public engagement ought to be doing. This paper reflects on these heterogeneous meanings and agendas through an analysis of focus group data from research on public engagement in stem cell research. Three broad visions of public engagement are identified: as education and information provision; as dialogue; and as participation in policy making. In analysing the implications of these visions three dimensions are highlighted: weakly and strongly structured visions of public engagement; the co-production of roles and relationships; and the framing of what is at stake. Each of these has the potential to include or exclude some groups in public engagement. We conclude that social scientists should seek to maintain the plasticity of public engagement as a necessary condition for greater participation and reflexivity in science policy, practice and governance.