Public engagement (PE) is part of the contemporary landscape of health research and innovation and considered a panacea for what is often characterised as a problem of trust in science or scientific research, as well as a way to ward off actual or potential opposition to new developments.  In this chapter we provide a synthesis of current conceptualisations of PE. We then consider what kinds of publics are ‘engaged with’ and what this means for the kinds of information exchanges and dialogues that are undertaken.  Different forms of PE ‘make up’ different kinds of publics:  engagements do not, indeed cannot, start with a clean sheet – neither with a pure public nor through a pure engagement. As Irwin, among others, has noted, PE is a political exercise and this wider context serves to frame what is engaged about.  It is therefore all the more important to reflect on the practice of PE and what it is hoped will be achieved.   We argue that clarity and transparency about the intention, practice and impact of PE are required if PE is to provide an authentic and meaningful tool within health research governance.