This report summarises the results obtained in the ASSURANCE project (EU contract number ENV4-CT97-0627). Seven teams have performed risk analyses for the same chemical facility, an ammonia storage. The EC's Joint Research Centre at Ispra and RisøNational Laboratory co-ordinated the exercise and led the comparison of results in order to reveal the causes for differences between the partners' results. The results of the project point to an increased awareness of the potential uncertainties in riskanalyses and highlight a number of important sources of such uncertainties. In the hazard identification phase it was revealed that the ranking of hazardous scenarios by probabilistic and deterministic approaches could result in completely differentconclusions. On the other hand, despite a large difference in frequency assessments of the same hazardous scenarios, there was good consensus on the ranking among the adherents of the probabilistic approach. Breaking down the modelling of both frequencyand consequence assessments into suitably small elements and conducting case studies allowed identifying root causes of uncertainty in the final risk assessments. Large differences were found in both the frequency assessments and in the assessment ofconsequences. The report gives a qualitative assessment of the importance to the final calculated risk of uncertainties in assumptions made, in the data and the calculation methods used. This assessment can serve as a guide to areas where, in particular,caution must be taken when performing risk analyses.