Risk analysis of nuclear power generation often requires the use of expert opinion to provide probabilistic inputs where other sources of information are unavailable or are not cost effective. In the Reactor Risk Reference Document (NUREG-1150), a methodology for the collection of expert opinion was developed. Earlier criticisms pointed out the need to establish principles for the collection and use of expert opinion. These principles include selection of experts to promote diversity of opinion, the use of state-of-the-art methods of probability elicitation including debiasing training, communication of findings through complete and clear documentation, and the preservation of the inherent uncertainty in the findings. The resulting methodology involves a ten-step process: selection of experts, selection of issues, preparation of issue statements, elicitation training, preparation of expert analyses by panel members, discussion of analyses, elicitation, recomposition and aggregation, and review by the panel members. These steps were implemented in a multiple meeting format that brought together experts from a variety of work places. The elicitation of the experts’ opinions was performed by teams versed in decision analysis and in the particular aspects of power plant safety being investigated.