This thesis is a discussion of the question words of not knowing: who does not know what and how do they not know it. It is also a discussion of issues at the public interface of science – expertise, democratic participation, and policy-relevant science – seen through the lens of non-knowledge. Shifting the attention to non-knowledge requires at least two uneasy departures from mainstream thinking; it requires the problematization and re-conceptualization of both the known-unknown nexus, and the science-policy nexus. Non-knowledge is presented in its full due as a social fact and with sensitivity to its complexity and socially constructed character. As such, it is a unique entry point into thinking about issues that have long been conceptualized in terms of knowledge, its authority and legitimacy, and those who have it. Focusing on non-knowledge reshuffles the entrenched lines of authority and the established ways of conceptualizing democratic expertise inscience policy, and points future inquiry into new directions.