Quantified risk assessment (QRA) is a technique for aiding decision making. In the major hazard area, the HSE uses the technique to formulate advice to local planning authorities on the siting of such installations or on the development of land in the vicinity of existing sites. The advice is based on the predicted variation of individual risk with distance and, in some cases, on estimates of the levels of societal risk. Such estimates are sensitive in varying degrees to the levels of uncertainty in the assumptions, judgements, data and models employed in the various stages of the QRA process. The risk estimates are calculated by a risk assessment tool, RISKAT. This paper describes the development and application of RISKAT and the research sponsored and conducted by the HSE to reduce uncertainty and improve the quality of the risk estimates. Reference is also made to related research, particularly that sponsored by the Commission of the European Communities (CEC).