Comparative risk assessment (CRA) is playing an ever-increasing role in environmental policy priority setting, as manifested in national and numerous subnational comparative risk projects. It is widely accepted that public values, interests, and concerns should play an important role in CRA. However, the philosophical basis for public involvement in CRA has not been adequately explored, nor have comparative risk projects always made explicit their rationales for public involvement. The author examines the political, normative, and epistemic rationales for public involvement and explores the case for public involvement in the making of ethical, valuation, and evidentiary judgments. The author considers key issues each rationale raises, problems eachfaces, and some of the implications of each for specific aspects of CRA.