Recent years have seen growing worldwide discussions, experiments, and expectations around various kinds of public engagement in the biosciences. This is especially so, in the governance of biotechnology—in research policy, risk regulation, and adoption of new innovations. How one defines public engagement necessarily affects the course of political, media, and civil society debate on these issues. Yet critics and even some proponents often misunderstand underlying rationales and imperatives for engagement. Strong opposition persists on the part of some policymakers in the ostensible name of science, even to the most modest forms of citizen participation in decisions about regulation or research. Where dialogue is supported between scientists, policy makers, stakeholders, and members of the public, it is often for contrasting reasons — reflecting motivations of some leading figures in science governance to control, as much as respect, contending public interests. Prominent experts have questioned whether ordinary people have the right or even the ability to engage on complex technical issues. Attempts to include stakeholders are criticized as slowing down innovation. Some scientists fear that irrational anxieties over particular issues mean that public engagement will lead to indiscriminately technophobic or anti-science results. How might we interpret these attitudes and controversies and better understand why public engagement matters? What are the practical policy consequences?