This is an extensive survey and critical examination of the literature on the use of expert opinion in scientific inquiry and policy making.Cooke considers how expert opinion is being used today, how an expert's uncertainty is or should be represented, how people do or should reason with uncertainty, how the quality and usefulness of expert opinion can be assessed, and how the views of several experts might be combined. He argues for the importance of developing practical models with a transparent mathematical foundation for the use of expert opinion in science, and presents three tested models. Detailed case studies illustrate how they can be applied to a diversity of real problems in engineering and planning.