This article examines social science relevant to public engagements and identifies the challenges to the goal of meaningful public input into science and technology policy. Specifically, when considering “which forms, features, and conditions of public engagement are optimal for what purposes, and why?” we find social science has not clarified matters. We offer a model to guide systematic research that defines and empirically connects variations in features and types of public engagement activities to specifically defined variations in effective processes and outcomes. The specification of models, as we have done, will guide policy makers, practitioners, and the public in determining what kinds of engagement techniques are optimal for what kinds of purposes. Our model is presented to start conversations and inspire research that in the future should help to ensure meaningful public participation that meets the promise of contributing thoughtful societal values and perspectives into governmental policies impacting science and technology research.