This book presents a fascinating analysis of expertise and policy formation, based on an in-depth study of the UK Royal Commission on Environmental Pollution. The Commission provided expert advice to governments from 1970 to 2011. Often portrayed as a 'scientific body', it was in fact an interesting hybrid, which embodied wide-ranging expertise. It delivered thirty-three reports, leaving a significant mark on British environmental policy, and having influence within Europe and beyond. Drawing upon an extensive literature and a wide range of sources, Knowledge, Policy, and Expertise provides the only full account of this important advisory body, covering a period in which the policy landscape was profoundly transformed. It offers a rich and detailed analysis of authority, autonomy, and trust; of the diverse roles that advisors can play and the networks within which they operate; and of the 'circumstances of influence' in which expert advice comes to be accepted gratefully, used strategically, absorbed in diffuse ways, or ignored. Above all, this book demonstrates the complexity and contingency of knowledge-policy relations, contributing substantially to a theory of expertise, and drawing out important implications for the future of 'good advice'.